There's nothing wrong with you. You just need to get out more and meet more people. Especially since you're in a small department, it's probably best to try to meet more people outside of your department. There are lots of threads on these fora that address this topic. This one, for example:
http://chronicle.com/forums/index.php/topic,52175.0.html It's focused on LGBT stuff, but many of the same points apply to anyone. Instead of going to parties where you don't know anyone, do volunteer work or join community organizations or pursue some kind of athletic/artistic/political/charitable activity that exposes you to new people. Follow up on those relationships: some of them will blossom into real friendships.
Above all, just get yourself out of the house/library and away from the books for an hour or two each day. It'll do you a world of good. Social isolation is not a necessary characteristic of grad school: it's just a characteristic of many grad students who are letting grad school dominate their lives.