The Immaculate Conception is "the unique privilege conferred on the Blessed Virgin, whereby she was preserved from original sin from the first moment of HER existence in view of the merits of her Divine Son."The new covenant was underway, our redemption and salvation through Jesus Christ is being arranged with the cooperation of a single young maiden with so much trust in God to change the course of history. One word from this young lady and God's plan is permitted to continue on.
With all the pressures of the day as a young unmarried, but betrothed woman, Mary agreed to God's plan. She wasn't completely sure how it would all play out, as her reaction to the angel's greeting tells us. The Angel was sent from God.....he said, "Hail, full of grace! The Lord is with you." But she was greatly troubled at what was said and pondered what sort of greeting this might be." So much so, that the angel was compelled to reasure her "not to be afraid, Mary, for you have found favor with God."
A woman is asked to fulfill a destiny that will take a level of bravery, trust, and submission at the same time. This is not the job of a "take charge' personality, it is that of a submitting personality to another who IS in charge. And as time went on, without knowing where the next requirement would come from, she followed on. Carrying out the tasks required to fulfill the mission, THAT is what our Mary said yes to. Yes, I will bare a God-child, name Him Jesus, and follow you till the fullness of time when your plan is completed and I have completed my tasks. No small mission, but a privilege in deed!