Let me share what St. Ambrose has to say in the Ember Wednesday Office at Matins, based upon the Annunciation pericope of Luke 1.
"The Divine Mysteries are indeed hidden; according to the prophet's dictum, no man can readily know God's planning. But still, from certain deeds and precepts of our Lord and Savior we can understand that there were very weighty reasons why she who was specially chosen to give birth to the Lord should be betrothed to a man. And why was her time not fulfilled before she was betrothed? Perhaps to keep anyone from claiming that she had conceived as a result of adultery.
'And when the Angel had come to her...' Recognize the virgin by her conduct; recognize the virgin by her modesty; learn from her words; learn from the mystery. It is natural for virgins to be troubled and hesitant at every approach of a man, to be fearful, feel anxious whenever addressed by men. Let women study to imitate this example of modesty. Mary was alone inside, unseen by any man. Alone there, with no companion, no witness, only the Angel found her, lest any trivial talking disturb the angelic salutation.
The mystery behind this mandate was so great that it was not given forth from the mouth of men, but was spoken by an Angel. This is the first time that any ear has heard the words 'The Holy Ghost shall come upon you.' They are heard, and they are believed. Then she speaks, 'Behold the handmaid of the Lord; be it done to me according to thy word.' Look at her humility, her readiness to serve! Forming no high opinion of herself despite the unexpected promise, she who is chosen to be the Lord's mother proclaims herself His handmaid."