I used to play alone but usually within a guild system or similar for some chatting/communication. Recently I've been playing a bunch of different games with the same people and it's nice to have someone to talk about the game with whilst playing, because if I try to talk to my husband about games he just stares at me blankly. When I lose my gaming buddy I'll probably feel a little lonely for a while, stuff is definitely more fun if you have someone to enjoy it with.
Playing with someone who's not at the same pace as you is super frustrating I think. I only have one friend who will actually level/play at the same sort of pace and rate as me, if the option was having to play super slowly or play alone, I'd probably choose alone.... but if you find someone who's actually in sync with you when it comes to gaming then it's pretty awesome.