Why do I find it hard to have relationships (platonic and non)?

Recently, I've became (depressingly) aware that I have no close friends. I have maybe 2 friends that I'm very close with, a guy and a girl. I particularly resent the girl because I feel as if our friendship is unhealthy, but that's beside the point. I'm alone at home with only my mom right now, while my friends are probably out partying. This saddens me as they have a wide range of friends, and while I'm acquaintances with most of them, I'm not particularly close with any of them. It really bothers me that I can't find close friends because sometimes I feel left out that they have ANOTHER group of friends that don't include me, and I only have them. It makes for some lonely times. It it a psychological thing or what?