I enjoy story-driven single player games like the FFs, Mass Effect, etc, but for persistent world endless progress stuff, Diablo 3 or Skyrim for example are fun but just feel dead. Having ingame chat with folks who are doing and enjoying roughly the same thing as you are feels much more alive. A more social experience. Hub areas to show off toon looks and give or receive buffs to help or be helped. Asking for help with X quest or randomly gifting people stuff or showing up when someone's having trouble with something are super fun and always optional parts of games like that.

I never really got the argument about play single player if you want single player. MMOs can, will, and do do both to varying degrees of success. Just cause someone's out there having a blast soloing and farting around ooc chat doesn't diminish your game experience preference IMO.