I was in the exact same position a few months ago. I feel better (though not great) now, but it's not clear to me exactly why. My introspective powers on this topic have been pathetic, but I can at least list for you what's changed since I felt the way you seem to:

1) I've started a fairly standard job and 2) moved in with my girlfriend. She's more social than I am, so chatting at the office, and then again at home with occasional guests (and her, of course) may be the answer. Or possibly, the mental shift I've made from being very uncertain about the sort of career I want and trying to stay ambitious enough to keep those options all open. Zed's mention of "you spend much time on intellectual issues" and "you're not producing, you're almost exclusively consuming intellectual stuff" hit close to home for me, there.