The wisdom of God reaches from end to end mightily and it arranges all things sweetly. Everything has its place in the pattern of divine providence. God fits everything and everyone to its place in His plan for the world. When God the Father resolved to send His only-begotten Son into this world to be born of a woman, He chose and fashioned a woman to be the worthy mother of so great Son. When we meet a very good child, we are quite likely to say, "He must have a very good mother". We explain the child goodness by the goodness of his mother. We believe that it is the mother's goodness which has made the child good. But in the mystery of the Incarnation, it is contrary which is true. Mary of Nazareth is the perfect Mother of God because her Son, Who is God, has made her so. Because the Son of God is all holy and perfect, Mary, His Mother in the flesh, must be as holy and perfect as she can be. God did not cast His eye over all the women in the world, seeking to discover which one was best fitted to be the mother of His Son. Mary was not chosen by God because he found her to be the most perfect of women. Rather, He made her the perfect woman because He had chosen her to be the Mother of His Son.
From "My Way of Life - Simplified Summa for Everyone"