I feel this way a lot. I work (almost) full time, dealing with coworkers and customers all day, doing a lot of VERY physically demanding work, and by the time the day's over... I'm fucking burnt. I don't have the energy left to talk to anyone (and if you've seen any of my posts, I talk just about as much as I type. But I just can't.)

Luckily I recently moved into my own place and can enjoy my solitude. Until it gets lonely. Then I want to start being social again and it doesn't work out, so then I get depressed, which ends up in me being isolated again... and it gets BAD.

So I think the reason you isolate should be because you're just a little tired of people right now.