From Little Pictorial Lives of the Saints:
This great festival takes its name from the happy tidings brought by the Archangel Gabriel to the Blessed Virgin, announcing the Incarnation of the Son of God. It commemorates the most important embassy that was ever known, an embassy sent by the King of kings, and performed by one of the chief princes of His heavenly court, and directed, not to the great ones of this earth, but to a poor, unknown virgin who, being endowed with angelic purity of soul and body, and perfect humility and submission to God, was greater in His eyes than the mightiest monarch in the world.
When the Son of God became man, He could have taken our nature without the cooperation of any creature; but He was pleased to be born of a woman, the One announced in the third chapter of Genesis. In choosing Her whom He raised to this most sublime of all dignities, He was turning to the one maiden who, by the riches of His grace and virtues, was of all others the most holy and the most perfect. The purpose of this embassy of the Archangel was to give a Saviour to the world, a victim of propitiation to the sinner, a model to the just, a son to this Virgin who would remain a virgin, and a new nature to the Son of God — the nature of man, capable of suffering pain and anguish in order to satisfy God’s justice for our transgressions.
When the Angel appeared to Mary and addressed Her, the Blessed Virgin was troubled; not at his coming, says Saint Ambrose, for heavenly visions and conversation with the blessed spirits had been familiar to Her, but what alarmed Her, he says, was the Angel’s appearing in human form, in the shape of a young man. What added to her alarm on this occasion was his words of praise. Mary, guarded by her modesty, was in confusion before expressions of this sort, and dreaded even the shadow of deluding flattery. Such high commendations made her cautious, until in silence She had more fully considered the matter: “She deliberated in her mind,” says Saint Luke, “what manner of salutation this could be.”
The Angel, to calm her, said: “Fear not, Mary, for Thou hast found favor before God.” He then informed Her that She was to conceive a Son whose name would be Jesus, who would be great and the Son of the Most High, and possessed of the throne of David, Her illustrious ancestor. Mary, out of a just concern to know how she may comply with the will of God without prejudice to Her vow of virginity, inquired, “How shall this be?” Nor did She give Her consent until the heavenly messenger informed Her that it was to be a work of the Holy Spirit, who, in making Her fruitful, would not alter in the slightest Her virginal purity. In submission to God’s will, without any further inquiries, She expressed Her assent in these humble but powerful words: “Behold the handmaid of the Lord; be it done unto Me according to thy word.” What faith and confidence Her answer expressed! What profound humility and perfect obedience!
Reflection. Humility is the foundation of a spiritual life. By it Mary was prepared for the extraordinary graces and virtues which would ever enrich Her, and for the eminent dignity of Mother of God.