"Hell is other people."
My current game addiction is WoW. It's explicitly multiplayer, although you can pretend ("the illusion of single-player") that you're playing by yourself for a lot of the play. Until someone ganks you, or starts spamming inane bullshit in the yell channel, or you have to go into a pick-up raid to accomplish something (damn legendary cloak quests).
In that latter case, you run into the worst of people, all in a little 10-player or 25-player microcosm. Narcissistic douchbags, trolls (some of whom are actually trolls), lazy asses who expect you to carry them, clueless weiners who don't understand the fight and can't be troubled to learn...
Too bad for me I actually enjoy the game, except for the parts where the "multiplayer" part ruins the rest.