Why do you think Peter was so emphatic and yet so wrong about denying Jesus?

Peter loved Jesus so much that he could never foresee denying Him. He believed in Jesus, the Son of God, and felt the Son of God could not be harmed by people. He was so emphatic because he wanted to prove his great love for Jesus.

But he had not fully comprehended the depth of Jesus taught, and the sacrifice He was going to experience: The sufferings, pains, humiliation, and death by crucifixion as an ordinary human being; separation from God and Resurrection, to redeem us from our sins, according the Lord’s plan.  

Peter as human being, and when faced with harm, he did deny Jesus, as stated by Jesus: “The spirit indeed is willing, but the flesh is weak."

Nevertheless; thereafter and realizing his act, Peter became one of the biggest both preachers and contributors to foundation of the Christian Church, also giving his life as tribute to his faith and beliefs.