God surprised us all when He, through the angel Gabriel, announced to the whole world the good news of the Incarnation. The surprise was beyond our imagination because God asked a quiet country girl, at a time and place that no one expected, to be the Mother of Jesus. It was good news for Mary and for the entire world and yet Mary was unable to welcome it at first because it was so sudden and she herself was unprepared. Indeed, there would never be enough preparation for such great news and Mary found it hard to comprehend that she had been chosen to bear the Son of God. Her 'yes' answer was the sign of trusting God more than understanding of the message and The Magnificat reveals how her mind responded.
My soul proclaims the goodness of the Lord and my spirit exalts in God my Saviour because He has looked upon his lowly handmaid... for the Almighty has done great things for me, holly is His Name. Lk 1,56ff
Mary answered 'yes' simply because of her pass experiences in the loving God. She had experienced through her life great things that the Lord has done for her and this time was no exception. She didn't understand fully what would be involved in being the Mother of the Lord and its consequences but she answered 'yes' because she loves God confidently, fully trusting that the Lord would do good things for her and now through her, other people would receive good things from the Lord. Saying 'yes' to the Lord is an acceptance to enter into the unknown future. Mary entrusts her entire life into God's plan of salvation for mankind by taking on the adventurous journey of being the Mother of Jesus which would bring both joy and sacrifices. Mary had been faithful till the end of her calling to be the Mother of the Lord.
The affirmation comes from the angel Gabriel: 'nothing is impossible to God' reconfirms Mary that it is God who does the mighty work. The Incarnation is the work of the Lord and Mary simply says 'yes' in response. She has taught her children that when we are willing to say yes to the Lord, God alone will take care of everything else. What we need is an open heart and the Spirit will slowly unfold the mystery through events of our lives.
Furthermore God has never asked us to carry out the mission which is beyond our capacity because God's grace is enough for us to fulfil the task assigned.