We have established from the Scriptures that for anything God desires to accomplish on earth, He would need a willing human vessel to do so - because He is a spirit. He does not have a body.
To send His Son into the earth - to realise what had been prophesied 700 years ago by prophet Isaiah (Isa.7:14) - the vessel must be pure and holy.
Angel Gabriel's greetings established that by addressing Mary as one "full of grace, the Lord is with you.' (Luke 1:28) "You will be with child and give birth to a son, and you are to give him the name Jesus. He will be great and will be called the Son of the Most High. The Lord God will give him the throne of his father David." (Lk.1:32) That established the child's royal lineage.
"How would this be since I am a virgin?" was Mary's natural reaction to this breaking news before Gabriel enlightens her. "The Holy Spirit will come upon you, and the power of the Most High will overshadow you, so the holy one to be born will be called the Son of God." (LK.1:35)
Young Mary needed sufficient grace to understand what was happening to her. At that point Gabriel broke the miraculous news of Elizabeth's pregnancy in her old age to her to confirm what God can do.
"O the depth of the riches both of the wisdom and knowledge of God! How unsearchable are His judgments and His ways past finding out." (Rm. 11:33)
Who can question God's ways of doing things? He reigns supreme.
