Here's one of my reasons. It's because people stick together in their little cliques like its life or death, and it's very hard for newcomers to get inside. Also, I'm quite paranoid, always saying to myself "should I call/email/text this new person I talked to or will s/he think I'm weird? Should I wait for them to contact me? Do they even like me or were they just trying to be nice and make small talk?" I end up waiting, nothing happens, and by that time I'm too embarrassed to even try anything by then. I completely suck at small talk. It is an activity I despise and try to avoid at all costs. When I do get into it, I end up making an utter fool of myself. I tangle my words. I'm only good at talking about things I like, but seriously, it's not easy bumping into someone in your most frequented geographical region with the same interests as you, unless you're lucky. I find it hard to communicate orally and I used to freeze up in high school when I had to make speeches in English class, for god's sake. I'm better now, but haven't taken near enough steps to get anywhere near towards making friends.
*opens up another beer and drinks forlornly*
I feel that many people I know have a very low criteria of what a friend is, anyway. They think a friend is just some social 'connection' who they see every day at work or something, and can scab party invitations off. Personally, I only count someone as a friend when I can talk to them about feelings and sappy shit like that that's pretty taboo in civilised conversation. But I digress.