I like both; sometimes I'm in the mood for one or the other. Take this table, for example:
-Doesn't require the kind of balance that Multiplayer does, and thus I can both powergame and not feel bad about it!
-Sometimes all I want to do is shoot fish in a barrel, and Single Player FPSes are exactly what the doctor ordered.
-There are those days where you just don't want to put up with other people's bullshit.
-Nothing is more satisfying than splattering some asshole's E-Brains all over some E-Wall with my E-Bullets.
-People, on average, are much smarter than AI's, which makes for a more rewarding experience.
-Pissing people off on Xbox Live is hilarious.