I ask myself the very same question all too often. Along with why do I belong to any particular crusing/sex sites. And while I can look at sex and think 'It's just sex' I can never seem to relax enough to enjoy the casual sex I occasionally go looking for, at least with picking up strangers. In theory *ahem* I've not had a problem with casual sex with friends though. Someone I know says something along the lines of it being good to fuck your friends or something like that. That too for me lately though is a distant memory while I try to 'recover' from a heinous past couple of years.

Don't be hard on yourself. Let your friends be hard in (no) with (er) on you. Rely upon your friends!