Well, we all have phases during which we don't feel like seeing anyone. I have them, and a friend of mine has been known to literally disappear for a few days without warning (usually causing us to worry out of our minds), because she couldn't deal with anyone during that time. We've learnt to accept that about each other.

And we also accept that sometimes, one of us will be depressed and it doesn't have anything to do with the others, hence they shouldn't take it personally. Making that clear to your friends might help avoid any misunderstandings, and will allow them to better deal with any melancholy you might be going through. And, well, while a bit of isolation can be good, exposing yourself to other people and new experiences usually takes your mind off your worries, if only for a little while. But you seem to already know that.