Why did God choose Mary? The Blessed Virgin Mary, the Mother of God, was conceived full of sanctifying grace and free of any stain of original sin. In Catholicism, this is called the Immaculate Conception. She was conceived in this way through the merits of Jesus Christ: by His sacrifice, she was preserved from original sin.

Furthermore, and strictly owing to a special privilege from God, she lived a life entirely free from sin, and it seems that she could not have done so otherwise:


  If any one say that man once justified can during his whole life avoid all sins, even venial ones, as the Church holds that the Blessed Virgin did by special privilege of God, let him be anathema."


First, a minor clarification. By "special privilege", does that quotation mean the very fact that Mary was born without original sin, or is it a separate gift?



I have heard of (or can imagine) some explanations for why Mary merited the reception of her graces, such as:


because of her perfect devotion to the Lord, or because of a natural disposition to be perfectly devoted to the Lord.
because she was chosen by God, and that is merit enough.
because she was Mother of God, and no other woman could have been chosen in her place, since no other woman was Mary.
we do not know, because this was never revealed.


But without the backing of what may be a fair bit of research, all of these options are idle speculation. According to the Catholic Church, why did God choose Mary, from all eternity, to be the Blessed Mother of His Son?
All Nations Call Her Blessed

Mary has been one of the most common names for women throughout the last couple thousand years for good reason: this is the name of the Mother of God. She has been called the God-bearer (Theotokos), the Blessed Virgin/Mother, the Mater Dolorosa, the Second Eve, the Queen of Heaven, the (Black) Madonna, Notre Dame, and the Woman for All Seasons. Her name has been sung on the lips of numerous choirs (Ave Maria), and she has been portrayed in art more than any other woman in history.1 She is a symbol of hope and liberation for many nations. Concerning Hispanic culture, one distinguished commentator has observed that 


  ...the Virgin of Guadalupe symbolizes the entire coherence of Mexico,
  body and soul.... [Her] image ... has become the unofficial, the
  private flag of Mexicans.2


Writing c. 305 A.D., St. Methodius extolled her, saying:


  Hail to you for ever, you virgin mother of God, our unceasing joy, for
  unto you do I again return. You are the beginning of our feast; you
  are its middle and end; the pearl of great price that belongest unto
  the kingdom; the fat of every victim, the living altar of the bread of
  life. Hail, you treasure of the love of God. Hail, you fount of the
  Son's love for man. Hail, you overshadowing mount of the Holy Ghost.
  You gleamed, sweet gift-bestowing mother, of the light of the sun; you
  gleamed with the insupportable fires of a most fervent charity,
  bringing forth in the end that which was conceived of you before the
  beginning, making manifest the mystery hidden and unspeakable, the
  invisible Son of the Father— the Prince of Peace, who in a marvellous
  manner showed Himself as less than all littleness. Wherefore, we pray
  you, the most excellent among women, who boastest in the confidence of
  your maternal honours, that you would unceasingly keep us in
  remembrance. O holy mother of God, remember us, I say, who make our
  boast in you, and who in hymns august celebrate the memory, which will
  ever live, and never fade away.3


Kings and queens have looked to her for comfort and guidance. Mary represents "the unbreakable link between Jewish and Christian history, between the First Covenant within which she was born and the Second Covenant to which she gave birth."4 All generations have indeed called her blessed.5

What Makes Mary So Special?

Luke points out in his Gospel that St. Joseph was of the house of David.6 But theologians and scholars have speculated through the centuries why this was of importance to the narrative, since Jesus was conceived without the seed of Joseph. It is agreed that it is important for Jesus to have this lineage in order to fulfill prophecy as the Jewish Messiah (Christ), but does this 'count' if the lineage is reckoned through the human father, when Jews generally consider the Jewish faith to be matri-lineal and Joseph was not involved in the conception? This led some early Fathers and scholars to propose that Mary is also of the house and lineage of David.7

Early Church Fathers also saw Mary foretold in the Old Testament, especially in the woman of the Song of Solomon (which is where the title 'Black Madonna' comes from) and in the woman of virtue/valor in the book of Proverbs.8 St. Jerome even saw the Song of Solomon 4:12-13 as evidence of Mary's perpetual virginity:


  That which is shut up and sealed reminds us of the mother of our Lord
  who was a mother and a Virgin. Hence it was that no one before or
  after our Saviour was laid in his new tomb, hewn in the solid rock.
  And yet she that was ever a Virgin is the mother of many
  virgins.9


But why was she chosen? Pius IX decreed concerning the Blessed Virgin Mary (and was partially quoted earlier),


  All are aware with how much diligence this doctrine of the Immaculate
  Conception of the Mother of God has been handed down, proposed and
  defended by the most outstanding religious orders, by the more
  celebrated theological academies, and by very eminent doctors in the
  sciences of theology. All know, likewise, how eager the bishops have
  been to profess openly and publicly, even in ecclesiastical
  assemblies, that Mary, the most holy Mother of God, by virtue of the
  foreseen merits of Christ, our Lord and Redeemer, was never subject to
  original sin, but was completely preserved from the original taint,
  and hence she was redeemed in a manner more sublime.10


Simply put, God found favor with her and blessed her by His grace.

Further Speculation

To further speculate, we can also look at her parents, Joachim and Anna. Early theologians and scholars have speculated that Mary was born to Joachim and Anna as a result of their fervent prayers in their old age.11 

Catholic Christians still say these words in the liturgy on the Nativity of Mary:


  Your nativity, O Virgin, has proclaimed joy to the whole universe. The
  Sun of Righteousness, Christ our God, has shone from you, O Theotokos.
  By annulling the curse he bestowed a blessing. By destroying death he
  has granted us eternal life. (Troparion)
  
  By your nativity, O most pure virgin, Joachim and Anna are freed from
  barrenness; Adam and Eve from the corruption of death. And we, your
  people, freed from the guilt of sin, celebrate and sing to you: The
  barren woman gives birth to the Theotokos, the Nourisher of our Life.
  (Kontakion)


The Catholic Church has historically taught that Mary was presented to the temple and had made a vow of virginity (which is the reason for her protest to the angel in Luke 1:34).


  As Joachim belonged to the royal family of David, so Anna is supposed
  to have been a descendant of the priestly family of Aaron; thus Christ
  the Eternal King and Priest sprang from both a royal and priestly
  family.12


Conclusion

From a Catholic perspective, if God chose to preserve Mary from original sin prior to her birth, to what in her life can we point as a reason for choosing her? According to Pope Pius IX, "by virtue of the foreseen merits of Christ, our Lord and Redeemer." This is truly the answer given by the Roman Catholic Church.



1 Jaroslav Pelikan. Mary Through the Centuries: Her Place in the History of Culture. (New Haven, CT: Yale University Press, 1998), 2.

2 Richard Rodriguez. Days of Obligation: An Argument With My Mexican Father. (New York: Penguin Books, 1993), 16-20.

3 Methodius, "Oration Concerning Simeon and Anna On the Day that They Met in the Temple", XIV, translated by William R. Clark. From Ante-Nicene Fathers, Vol. 6. Edited by Alexander Roberts, James Donaldson, and A. Cleveland Coxe. (Buffalo, NY: Christian Literature Publishing Co., 1886) Revised and edited for New Advent by Kevin Knight. Available on New Advent.

Also cf. Philip Schaff. Ante-Nicene Fathers, Vol. 6: Fathers of the Third Century: Gregory Thaumaturgus, Dionysius the Great, Julius Africanus, Anatolius, and Minor Writers, Methodius, Arn. (Grand Rapids, MI: Eerdmans, 1885), XIV, 393. Publicly available on CCEL.

4 Pelikan, 25.

5 cf. Luke 1:48, where Mary sings, "Surely, from now on all generations will call me blessed." Some manuscripts of Luke 1:28 also record the angel Gabriel as saying, "Blessed are you among women" in addition to pointing out that she is "favored" by God and that God is with her.

6 cf. Luke 1:27.

7 Pelikan, 24-25. Specifically, cf. St. Augustine and later in history, Annius of Viterbo, who both made similar propositions concerning the lineage of Heli/Joachim/Eliachim.

8 Ibid., 25. cf. Song of Solomon 1:5, which in the Latin Vulgate is rendered 'Nigra sum sed formosa' ("I am black but comely"). Interestingly, the Hebrew text actually says that she is "black and beautiful" (not but), which is also indicated in the Septuagint ("Μέλαινά εἰμι καὶ καλή"). This has made Mary a powerful symbol for many non-white cultures, especially Hispanics.

Also cf. Proverbs 31:10, which in the Latin Vulgate reads 'Mulierem fortem quis inveniet?' ("The woman of valor, who will find?").

9 Jerome. Against Jovinianus (Book I). Translated by W.H. Fremantle, G. Lewis and W.G. Martley. From Nicene and Post-Nicene Fathers, Second Series, Vol. 6. Edited by Philip Schaff and Henry Wace. (Buffalo, NY: Christian Literature Publishing Co., 1893.) Revised and edited for New Advent by Kevin Knight, 31. Available on New Advent.

10 Constitution Ineffabilis Deus of December 8, 1854, under section entitled "Testimonies of the Catholic World." Available on New Advent.

11 Specifically St. John Damascene, St. Gregory of Nyssa, St. Germanus of Constantinople, and  St. Fulbert of Chartres. "The [apocryphal] Protoevangelium [of James] gives the following account: In Nazareth there lived a rich and pious couple, Joachim and Hannah. They were childless. When on a feast day Joachim presented himself to offer sacrifice in the temple, he was repulsed by a certain Ruben, under the pretext that men without offspring were unworthy to be admitted. Whereupon Joachim, bowed down with grief, did not return home, but went into the mountains to make his plaint to God in solitude. Also Hannah, having learned the reason of the prolonged absence of her husband, cried to the Lord to take away from her the curse of sterility, promising to dedicate her child to the service of God. Their prayers were heard; an angel came to Hannah and said: "Hannah, the Lord has looked upon thy tears; thou shalt conceive and give birth and the fruit of thy womb shall be blessed by all the world". The angel made the same promise to Joachim, who returned to his wife. Hannah gave birth to a daughter whom she called Miriam (Mary)." cf. New Advent article on Anne/Hannah. An astute reader cannot help but see the similarity with the biblical account of the birth of Samuel.

12 cf. Aug., Consens. Evang., l. II, c. 2