I judge people to find out stuff about them for my own use in my personal study of psychology, and I judge people to see how far I can take a relationship. I'm not going to spend days chasing a dead end trying to make friends with someone, if you know what I mean? I also judge on the basis of religion (Gasp) because I know that people have prejudices attached to their religions. And these prejudices affect your interactions with people. I tend to analyze a lot of factors, and one thing that's important to me is intelligence. It might seem harsh and as if I'm putting off a lot of people, and it might be that way. But I like to have someone that I can speak to on my own level, otherwise I just feel shut up inside myself without any way of expressing. When someone judges in an extreme, I think that boundaries tend to be overlooked. (Interesting that I should talk about boundaries, as I'm not too good at defining these myself, lol.) People have a right to their own opinions and ways of doing things, whether or not they're right or wrong. When not judging is taken to an extreme, you become a door mat I think, if you're the kind of person who makes your judgments externally seen.
Also, the title you chose is interesting, because it's merely a matter of cause and effect. If you judge someone else, of course they're going to want to get back at you and judge you too to feel vindicated :P.