I don't like shoehorned online multiplayer, otherwise I don't mind it. I'll readily play DOTA or TF2, but try to make something like any Mario platformer a multiplayer game, and I lose interest. And my Internet can get quite fussy when I do play those games sometimes, so there's something keeping me on singleplayer most of the times.
There's also the fact that I've never found a multiplayer experience that truly hung me by my balls and made me experience the game. I haven't found that awesome experience like Dark Souls for multiplayer.[footnote]I'm aware that there is some sort of multiplayer in Dark Souls, but I'm talking pure multiplayer, like in DOTA, COD, TF2, etc.[/footnote]