I'm not really into multiplayer and I'm not at all interested in free-to-play MMORPGs and such, but I'd disagree that it's no fun to play online with people you don't know from "real life". It all depends on the game, your company and the way you play. You can have lots of fun playing Neverwinter Nights online in co-op mode, for example. And the people you play with don't need to stay anonymous, even though you might not know their real life name. They're still people and you can get to know them, just like the folks on the forums here.

The reason why I still prefer Single Player most of the time is more due to my specific interests in gaming, e.g. story, dialogues, relaxed exploration. If you play with other people the game mechanics have to adapt. You won't enjoy a game as heavy on story and dialogues as Torment with a friend, because reading is not a very social activity. So most of the times these elements are cut down in Multiplayer and it's more about tactical combat, beating challenges together or against each other etc., and all of that can be fun but I don't enjoy it for long because I'm generally less interested in my character's levels, abilities or any scores than in the progress of an interesting story and the slow exploration of a setting.

This is where playing with people in the same room who all know each other comes into play. Many moons ago I played Unreal Tounament and Tribes with a clan made up of the guys in my platoon. When going into a CTF match against another guild we all knew what what expected of us and what we could expect from each other. As an added bonus I would sometimes take a squad of soldiers out into the desert for some special combat maneuvers in an area made to look somewhat like a UT map. Haviing practiced the same drills in both the real and digital world really paid off when in an actual game against another clan.
Except when it does. When core game mechanics of a series are changed or removed for the sake of multi player then it takes away from the SP experience. I can guarantee that somewhere in the game publishing world there is some jackhole who would love to get their hands on the rights to Amnesia so that they can make Amnesia 2 a multiplayer FPS because they know that the name alone will generate sales.