The question was posed in Catechesis on Sunday: Why Mary? Why did God choose her?
I think there are many ways to answer the question, but the one that came to mind first was this: because He knew it wouldn't destroy her.
Think of the overwhelming pride that would engulf us if an angel informed us that we had been chosen for such a task and such an honor! Our fat heads would be fatter than ever. But not she.
She marveled that God would so lift up the lowly, for that is how she continued to think of herself: as the lowly servant of the Lord, who consented to her Lord's bidding in faith. God chose a mother for His Son who was by God's unspeakable grace a truly humble person. Like her Son after her, she thought not of privilege and power but of undeserved favor and service.
O Blessed Trinity, lift us to the humility of the Mother of God!