So the very first thing Gabriel says to Mary is that she is about to receive a free bestowal of God's grace. She does not deserve this honor. It is grace. There are other virgins in Nazareth. God could have prepared them.
Grace eliminates all boasting.
Parents are prone to boast about their children. How much more if one of your children is the Son of God. So Gabriel quenches the spirit of pride before he does anything else. The Lord is with you, Mary, in a way you can't fathom. But never forget, it is a favor, a free gift of grace.