It's "other people", period, and it's not even restricted to foul mouthed brats or griefers. When you multiplayer game, you're essentially putting yourself in a situation where the quality of your recreation is entirely dependent on the behavior of a pack of total strangers. Racist Bob in the party? There goes your fun. Know-it-all Tom up in your shit? There goes your fun. Guild Princess causing drama again? There goes your fun. People keep going afk to have a smoke or walk their cat or pinch a loaf? There goes your fun. Your opponent rage quits or spews insults, hacks or griefs or just generally acts like a prick because he's angry you're beating him/not putting up enough of a fight? There goes your fun. Player silently and methodically waxes you without the slightest bit of a contest occurring because they've been honing their skills for years and you just bought the game yesterday? There goes your fun. And on, and on, and on, and on, and on, and on, and on.
Now consider that this is a hobby that was originally primarily migrated towards by introverts who don't deal well with social situations to begin with and aren't used to having to cope with the pressures annoying people can put on your patience and time, and you have a recipe for near constant misery punctuated by occasional glimpses of wary enthusiasm.
So, the answer is "other people". Can't live with em, still illegal to kill them and stack their bodies in your root cellar like cordwood.