Why do Christians believe Jesus needed to be both fully God and fully Man?

In the Bible it says that Jesus slept, ate breakfast, washed, wore clothes, chatted, cried, ate lunch, spent time with friends, drank and could read and write. Jesus could do all these things just like other people... but the Bible also says that Jesus is Lord. 

Christians believe Jesus was fully God and fully man. They believe He wasn’t part man and part God, He was man through and through and God through and through.

 
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Christians believe that to show what God is like Jesus needed to be God, but to teach people how to live He needed to be like people, He needed to be human.

The Bible also teaches that Jesus needed to be born perfect to make a way for everybody, everywhere to become close friends with God. Christians believe that to be born, Jesus needed to be a human and to be perfect He needed to be God's Son - Jesus needed to be fully Man and fully God.

However Christians also believe it is difficult to understand that Jesus is fully God and fully human too because only Jesus has ever been God and a man at the same time.