I'm definitely more of a singleplayer too. Even in multiplayer my preference is cooperative gameplay! hehe
SP is, as W7RE said, relaxing. It's convenient to hop into a game whenever I feel like it but MP is demanding. It needs organization and attention, Lord knows how many times I've seen people get frustrated and infuriated by the one player who's not paying much attention to the game, like being on the phone or having a conversation etc.
But you know what really "grinds my gears?" People who go online and into a game and do nothing but pick around for map exploits. These people are prevalent in L4D!! I appreciate the value of exploration and nit-picking occasionally but for the love of God do it on your own time in singleplayer mode! Geez! I have logged into so many servers where the game just doesn't go on because people are just doing stupid shit like that. I mean, play the f*'cking game!!
Sorry, I had to get that out.