I'm 23.
And A. it's hard for me to find friends, let alone something more; I'm a little too much for most guys to handle, and I'm picky. A guy who likes me, and especially one I like back, comes along only once in a blue moon. And I don't see the point in spending my life mooning over the fact that I can't find a guy (plus if I say I don't want a relationship, people will think I'm single because of that, not because I can't get a guy).
And B. I was hurt very badly in my last relationship, just absolutely screwed over in many ways and my life and future plans and heart were absolutely torn to pieces. Even prior to that I didn't believe that relationships last, I don't now even more than ever, and I really don't have the time nor the energy to keep having my heart broken again and again. I spent four years with my ex thinking we were deeply in love, deeply committed, and with us planning a life together; I only have so many years in my life and going through that over and over again is not my idea of fun. Plus, we had a fantastic relationship, and I know I'm not going to find anything that good again, and I don't want to settle for something less.
In short....I'd rather live for myself than get myself in a tizzy looking for a guy and then spending an entire relationship waiting for the axe to fall and picking up the pieces when it does.