ragefilledmuffin

I was thinking last night about how I used to judge people I barely knew. I'd put them in a category and that was it. If I decided I didn't them or they weren't "good enough", I pretty much avoided them.
I can see now that I was young and insecure. I was trying to make myself feel better by placing myself in some kind of hierarchy based on whatever criteria I was judging them on.
I was wrong about a lot of these people. Many of them are now good friends of mine or at least acquaintances whom I have a lot of respect for.
I do it less now but I still make snap judgements about some people. I wonder why this is. I've been wrong about three people I've met or gotten to know recently and that was what prompted my thinking about this subject.

Am I the only one who has done this?
Do you guys size people up, only to find out later you were wrong about them? I will add that I also have the opposite happen--I'll think someone is a pretty good person and they'll end up being a colassal jerk.
Am I just making my judgements with too little information? I know everyone judges to a certain extent, but I'm wondering if, in general, I'm too hard on people?
And, is this a sign of my immaturity/youth? Has anyone else experienced this but changed it over time?