Why does it seem when a undefeated boxer loses a fight they seem like a worser and not the same fighter?

Some times a really hard fight can age a fighter win or lose but usually it comes down to a loss of confidence. An underfeated fighter will often be overhyped and feel invincible once they lose its a mive blow to there confidence which can mivly affect there ability. Jeff Lacy is a prime example of this before losing to Calzaghe he was rated the best supermiddleweight in the world and a potential future hall of fame fighter. When he lost people wrote him off saying he was one dimensional and overrated etc and he never go his confidence back after that one loss.