The following paragraphs explain this doctrine.

The Church thus confesses that Jesus is inseparably true God and true man. He is truly the Son of God who, without ceasing to be God and Lord, became a man and our brother (469)

The unique and altogether singular event of the Incarnation of the Son of God does not mean that Jesus Christ is part God and part man, nor does it imply that he is the result of a confused mixture of the divine and the human. He became truly man while remaining truly God. Jesus Christ is true God and true man.  (464)

[T]he fifth ecumenical council, at Constantinople in 553, confessed that “there is but one hypostasis [or person], which is our Lord Jesus Christ, one of the Trinity. … He who was crucified in the flesh, our Lord Jesus Christ, is true God, Lord of glory, and one of the Holy Trinity.(468)

Because human nature was assumed, not absorbed, in the mysterious union of the Incarnation, the Church was led over the course of centuries to confess the full reality of Christ’s human soul, with its operations of intellect and will, and of his human body. In parallel fashion, she had to recall on each occasion that Christ’s human nature belongs, as his own, to the divine person of the Son of God, who assumed it. Everything that Christ is and does in this nature derives from one of the Trinity. The Son of God therefore communicates to his humanity his own personal mode of existence in the Trinity. In his soul as in his body, Christ thus expresses humanly the divine ways of the Trinity: The Son of God. . . worked with human hands; he thought with a human mind. He acted with a human will, and with a human heart he loved. Born of the Virgin Mary, he has truly been made one of us, like to us in all things except sin. (470)

Christ possesses two wills and two natural operations, divine and human. (475)

At the time appointed by God, the only Son of the Father, the eternal Word, that is, the Word and substantial Image of the Father, became incarnate; without losing his divine nature he has assumed human nature.  (479)

Jesus Christ is true God and true man, in the unity of his divine person; for this reason he is the one and only mediator between God and men. (480)

Jesus Christ possesses two natures, one divine and the other human, not confused, but united in the one person of God’s Son. (481)

Christ, being true God and true man, has a human intellect and will, perfectly attuned and subject to his divine intellect and divine will, which he has in common with the Father and the Holy Spirit. (482)

The Incarnation is therefore the mystery of the wonderful union of the divine and human natures in the one person of the Word. (483)

The RCC’s position is that Jesus must continue to honor his mother, therefore, intercession to Mary is a first step to talk to Jesus.  Furthermore, the theory is that in order for Mary to give birth to God she had to be sinless herself.  The flaw in this thinking is that in order for Mary’s mother to give birth to a sinless daughter, she too would need to be sinless.  However, this theory of Mary’s sinlessness causes many flawed doctrines related to Mary from the RCC.  They make Mary’s birth, life and death special like Jesus’. 

They believe that Mary was given special grace at birth so that she could later bear the Christ.  Therefore, her life is different because she is the only human to live a sinless life.  Then after Christ’s death they believe she took on a special ministry of prayer, because Jesus would listen to her prayer more then anyone else.  They then make her death different in that they say she, like Christ, ascended to heaven. 

See the paragraphs below that were referenced above:

The Virgin Mary is [faith’s] most perfect embodiment. (144)

Throughout her life and until her last ordeal when Jesus her son died on the cross, Mary’s faith never wavered.  She never ceased to believe in the fulfillment of God’s word.  And so the Church venerates in Mary the purest realization of faith. (149)

Mary stands out among the poor and humble of the Lord, who confidently hope for and receive salvation from him. After a long period of waiting the times are fulfilled in her, the exalted Daughter of Sion, and the new plan of salvation is established. (489)

The Christian tradition sees in this passage an announcement of the “New Adam” who, because he “became obedient unto death, even death on a cross”, makes amends superabundantly for the disobedience, of Adam.[305] Furthermore many Fathers and Doctors of the Church have seen the woman announced in the “Proto-evangelium” as Mary, the mother of Christ, the “new Eve”. Mary benefited first of all and uniquely from Christ’s victory over sin: she was preserved from all stain of original sin and by a special grace of God committed no sin of any kind during her whole earthly life  (411)

RCC believe Mary had some role in the decision of the conception of Jesus Christ.  RCC says she was invited to conceive Christ.  However, in the gospels she was told what things would happen, she was not given a choice or invitation. 

Mary was invited to conceive him in whom the whole fullness of deity would dwell bodily. (484)

The Virgin Mary cooperated through free faith and obedience in human salvation.  She uttered her yes in the name of all human nature.  By her obedience she became the new Eve, mother of the living. (511)

The paragraphs below show some of the depths that the RCC goes in explaining this doctrine of Mary.  These paragraphs show why it is that the RCC see Mary as worthy of almost worship.  She is placed as close to being God as a human can get.  To the point that in heaven we should look forward to not only seeing Christ but desire to see Mary too. 

What the Catholic faith believes about Mary is based on what it believes about Christ, and what it teaches about Mary illumines in faith in Christ. (487)

[Mary’s] spiritual motherhood extends to all men whom indeed [Christ] came to save. (501)

Mary, the all-holy ever-virgin Mother of God, is the masterwork of the mission of the Son and the Spirit in the fullness of time. For the first time in the plan of salvation and because his Spirit had prepared her, the Father found the dwelling place where his Son and his Spirit could dwell among men. In this sense the Church’s Tradition has often read the most beautiful texts on wisdom in relation to Mary. Mary is acclaimed and represented in the liturgy as the “Seat of Wisdom.” In her, the “wonders of God” that the Spirit was to fulfill in Christ and the Church began to be manifested: (721)

The Holy Spirit prepared Mary by his grace. It was fitting that the mother of him in whom the whole fullness of deity dwells bodily should herself be full of grace. She was, by sheer grace, conceived without sin as the most humble of creatures, the most capable of welcoming the inexpressible gift of the Almighty. It was quite correct for the angel Gabriel to greet her as the Daughter of Zion: Rejoice. It is the thanksgiving of the whole People of God, and thus of the Church, which Mary in her canticle lifts up to the Father in the Holy Spirit while carrying within her the eternal Son. (722)

In Mary, the Holy Spirit fulfills the plan of the Father’s loving goodness. With and through the Holy Spirit, the Virgin conceives and gives birth to the Son of God. By the Holy Spirit’s power and her faith, her virginity became uniquely fruitful. (723)

In Mary, the Holy Spirit manifests the Son of the Father, now become the Son of the Virgin. She is the burning bush of the definitive theophany. Filled with the Holy Spirit she makes the Word visible in the humility of his flesh. It is to the poor and the first representatives of the gentiles that she makes him known. (724)

Finally, through Mary, the Holy Spirit begins to bring men, the objects of God’s merciful love, into communion with Christ. And the humble are always the first to accept him: shepherds, magi, Simeon and Anna, the bride and groom at Cana, and the first disciples. (725)

At the end of this mission of the Spirit, Mary became the Woman, the new Eve (“mother of the living”), the mother of the whole Christ. As such, she was present with the Twelve, who with one accord devoted themselves to prayer, at the dawn of the “end time” which the Spirit was to inaugurate on the morning of Pentecost with the manifestation of the Church. (726)

We believe that the multitude of those gathered around Jesus and Mary in Paradise forms the Church of heaven. (1053)

If it was Mary’s faith that enabled her to assume this role and due to this role she was made sinless, it could be said that Mary merited her own sinlessness.  So how did Mary start becoming known as the mother of God?

The Council of Ephesus proclaim in 431 that Mary truly became the Mother of God by the human conception of the Son of God in her womb. (466)

The Council was arguing the Deity of Christ.  The phrase “mother of God” was not a reflection on Mary as much as it was a comment on Christ being God.  It was not stating that Mary was any different type of human being but that she was chosen to bear not just a man but a man who was God. 

To become the mother of the Saviour, Mary was enriched by God with gifts appropriate to such a role. The angel Gabriel at the moment of the annunciation salutes her as “full of grace”. In fact, in order for Mary to be able to give the free assent of her faith to the announcement of her vocation, it was necessary that she be wholly borne by God’s grace.  (490)

The following paragraphs explain this doctrine.

Mary is the symbol and the most perfect realization of the Church. (507)

Mary goes before us all in the holiness that is the Church’s mystery as the bride without spot or wrinkle.  This is why the “Maian” dimension of the Church precedes the “Petrine”. (773)

But while in the most Blessed Virgin the Church has already reached that perfection whereby she exists without spot or wrinkle, the faithful still strive to conquer sin and increase in holiness. And so they turn their eyes to Mary: in her, the Church is already the “all-holy.” (829)

[The RCC’s] holiness shines in the saints; in Mary [the RCC] is already all-holy. (867)

The Virgin Mary . . . is acknowledged and honored as being truly the Mother of God and of the redeemer…. She is ‘clearly the mother of the members of Christ’ “Mary, Mother of Christ, Mother of the Church.” (963)

Mary’s role in the Church is inseparable from her union with Christ and flows directly from it. This union of the mother with the Son in the work of salvation is made manifest from the time of Christ’s virginal conception up to his death; Thus the Blessed Virgin advanced in her pilgrimage of faith, and faithfully persevered in her union with her Son unto the cross. There she stood, in keeping with the divine plan, enduring with her only begotten Son the intensity of his suffering, joining herself with his sacrifice in her mother’s heart, and lovingly consenting to the immolation of this victim, born of her: (964)

After her Son’s Ascension, Mary aided the beginnings of the Church by her prayers. In her association with the apostles and several women, we also see Mary by her prayers imploring the gift of the Spirit, who had already overshadowed her in the Annunciation. (965)

the Virgin Mary is the Church’s model of faith and charity. … she is the exemplary realization of the Church. (967)

she is a mother to us in the order of grace. (968)

This motherhood of Mary in the order of grace continues uninterruptedly from the consent which she loyally gave at the Annunciation and which she sustained without wavering beneath the cross, until the eternal fulfilment of all the elect. Taken up to heaven she did not lay aside this saving office but by her manifold intercession continues to bring us the gifts of eternal salvation …. Therefore the Blessed Virgin is invoked in the Church under the titles of Advocate, Helper, Benefactress, and Mediatrix. (969)

“Mary’s function as mother of men in no way obscures or diminishes this unique mediation of Christ, but rather shows its power. … just as the priesthood of Christ is shared in various ways both by his ministers and the faithful, and as the one goodness of God is radiated in different ways among his creatures, so also the unique mediation of the Redeemer does not exclude but rather gives rise to a manifold cooperation which is but a sharing in this one source. (970)

By pronouncing her fiat at the Annunciation and giving her consent to the Incarnation, Mary was already collaborating with the whole work her Son was to accomplish. She is mother wherever he is Savior and head of the Mystical Body. (973)

We believe that the Holy Mother of God, the new Eve, Mother of the Church, continues in heaven to exercise her maternal role on behalf of the members of Christ (975)

The following paragraphs explain this doctrine.

Mary benefited first of all and uniquely from Christ’s victory over sin: she was preserved from all stain of original sin and by a special grace of God committed no sin of any kind during her whole earthly life. (411)

To become the mother of the Saviour, Mary was enriched by God with gifts appropriate to such a role. The angel Gabriel at the moment of the annunciation salutes her as “full of grace”. In fact, in order for Mary to be able to give the free assent of her faith to the announcement of her vocation, it was necessary that she be wholly borne by God’s grace. (490)

Through the centuries the Church has become ever more aware that Mary, “full of grace” through God, was redeemed from the moment of her conception. That is what the dogma of the Immaculate Conception confesses, as Pope Pius IX proclaimed in 1854: The most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Saviour of the human race, preserved immune from all stain of original sin. (491)

The splendour of an entirely unique holiness by which Mary is enriched from the first instant of her conception comes wholly from Christ: she is redeemed, in a more exalted fashion, by reason of the merits of her Son. The Father blessed Mary more than any other created person in Christ with every spiritual blessing in the heavenly places and chose her in Christ before the foundation of the world, to be holy and blameless before him in love. (492)

The Fathers of the Eastern tradition call the Mother of God “the All-Holy”, and celebrate her as free from any stain of sin, as though fashioned by the Holy Spirit and formed as a new creature. By the grace of God Mary remained free of every personal sin her whole life long.  (493)

… without a single sin to restrain her. (494)

… from the first instant of her conception, she was totally preserved from the stain of original sin and she remained pure from all personal sin throughout her life. (508)

She was, by sheer grace, conceived without sin as the most humble of creatures, the most capable of welcoming the inexpressible gift of the Almighty. (722)

But while in the most Blessed Virgin the Church has already reached that perfection whereby she exists without spot or wrinkle, the faithful still strive to conquer sin and increase in holiness. And so they turn their eyes to Mary: in her, the Church is already the all-holy. (829)

[T]he Immaculate Virgin, preserved free from all stain of original sin. (966)