Fighting is an outlet for pent-up emotions; because the Narrator lives in a highly repressed society, he has a lot of emotions bottled up. Where the rest of society celebrates
beautification and vacuous self-improvement, the Narrator just wants to hurt himself.
Self-destruction, he believes, is the answer to his problems.
Fight club becomes so popular because of its immediacy and visceral qualities: next to fight club, regular life feels like a lazy daydream.