LogFAQs > #911109988

A nonlethal fair duel or spar between two PCs can be done, and is fine. I've had plenty of those, they're fun.

That's often not what PvP is. PvP is often not a mutually agreed upon situation, done openly and fairly. It's often someone stealing from another party member. Or murdering him in his sleep. Or betraying the party in the middle of a fight when they're hurt and vulnerable.
At least IME, PvP is often opportunistic, craven, and self-serving. Hence why it's so despised.

In a tabletop game, you're placing a fair amount of trust in each other. You're roleplaying characters, but it's not actually you, so your character's not getting the subtle body language and voice tone cues that their "friend" might not be so friendly that would happen if it were truly two people spending all day around each other in real life.

Not to mention how almost always the party just meets in a tavern or something and never knew each other beforehand. Realistically, you probably shouldn't trust such people so readily and completely, but players forfeit that apprehension for the sake of not grinding the game to a paranoid halt. When someone attacks or cheats another player's character, he's essentially exploiting that goodwill and unearned trust for his own thrills/benefit, at the other player's expense.
It's pretty vile.