I kind of have a similar feeling, but it's more towards people who I've dealt with in the past. I tend to think each new person I meet who seems interesting is going to be a much better person, so much better than the last person I dealt with.
And then as I deal with the whole cycle of hearing rumors about them, having them talk about me, having them behave in ways that I consider unacceptable, and just generally seeing all their flaws, I get really disgusted with them, my patience wears thin, I keep trying to forgive them but end up being unable to truly do so, and eventually just move on and start over with a new person/group. Eventually it becomes intolerable to deal with the whole mass of people and common traits I now distrust and dislike in any given community, and then I have to move to a whole new place hoping things will be better, or just stay in the same place, resentful and miserable about the past, disliking everyone and generally just complaining about them all the time. I've done the latter for years at times, more often than I'd care to admit.
So the attitude you seem to have towards humanity in general... I have the same attitude, but towards the people I've known in the past and have been familiar with. I often have a very different attitude towards new people, and the very, very few who haven't let me down in those ways (like 3/100 or something).
Anyway, I think that perhaps you feel this way because you're projecting past traits onto new people. You aren't really seeing them for their good or positive potential, but you're seeing how they remind you of people from the past who let you down. You're not judging them, you're judging your idea of them that's informed by a lot of bad experiences with similar people. The problem is, people who are very similar in most ways can differ in some very important, key ways that make them surprisingly tolerable or surprisingly intolerable.
I guess what I've learned is that you have to give everyone a chance. Most of them will blow it, but you'll feel better about yourself for doing so, and you'll almost certainly find a few gems among all the junk. Think of it like digging around in a dumpster... most of what you find is filthy and disgusting, but once in a while you'll find something you treasure and wouldn't want to part with.