God gives graces according to his own will. And when he gives us grace in times of our needs upon our requests, it surely is grace. However, there are times that God gives grace apart from our will. We can call it 'Persistent Grace.' And this is the Mary's case in our passage. She never asked for it, or she did not even think about it, yet one day God sent an angel to her to deliver his message.
What the angel told her first was that she have found favor with God. And the reason is that she will give birth to the Messiah. To us who knows the result, it is a great favor. However, to Mary in that situation, it is a scary news than a good news, because she never knew how Joseph or her family would react. It was hard to see that as a good news from the beginning.
So Mary asked the angel that how could that be? She was trying to give a best objection indirectly. And angel answered to that not by giving her logical reasons to persuade her, but by pointing out God's power. Now this became a faith issue rather than an scientific or logical issue. And Mary responded with faith and accepted this grace, and became a woman who have found favor with God.
It is same to us. Sometimes God gives us persistent grace apart from our will and plan. But even though we may have to take a risk or even give up our best plans, we can trust God who is gracious and powerful. Christmas is a great example. No one dare to ask God to send his son for us. Yet, God sent Jesus to be born for us. This is the strong evidence that we can accept God's persistent grace with faith.