The miracle of God becoming human is amazing and unique to Christianity. In his incarnation, Jesus added full humanity to his full deity and entered into his creation. Light entered the darkness! It's important to recognize that Jesus is both 100% God and simultaneously 100% human. He doesn't switch between modes, but his two natures are eternally joined in his one person. Theologically this is described as the "hypostatic union." Jesus becomes human so that we can see the Father (and know exactly what God is like), see what humanity was created to be, and see who we are invited to become in Christ.
At Christmas we celebrate the fact that God became human. Jesus, fully and eternally God, forever added full humanity to his deity at the incarnation. He is fully God and fully human. The question is, "Why?"
