Put yourself in Gabriel's shoes:
...The first line -- "Hail, Mary, full of grace, the Lord is with thee" -- is drawn right from the angel Gabriel's words to Our Lady in the Annunciation scene (Lk. 1:28). To more fully appreciate the meaning of this opening line in the Hail Mary, imagine what these words originally meant to the Archangel Gabriel. Gabriel is an angel who existed long before Mary did. Gabriel has been around a lot longer than the nation of Israel or the entire human family. In fact, Gabriel was there when God first created the world. From the beginning of his existence, Gabriel has been worshiping, adoring, and loving the infinite, almighty God, the Creator: the Blessed Trinity.
And now, this great angel is sent to a little planet in the universe called earth . . . to a small, insignificant village called Nazareth . . . to a tiny little creature, a woman named Mary -- in order to announce to her that the all-holy, all-powerful God he has been worshipping from the beginning of his existence is about to become a little baby in her womb. In awe over that profound mystery of his eternal God becoming a little embryo in Mary's womb, Gabriel greets Mary saying, "Hail, full of grace, the Lord is with you" (Lk. 1:28). Indeed, the Lord has not been with anyone like He is about to be with Mary. In joyful wonder, Gabriel recognizes this, and his words give praise to God for becoming man in her.
Do read it.