I believe that people judge others because they lack confidence in themselves. somehow trying to make others feel bad about themselves makes them feel better. It's sad. So hold your head high & consider the source . These are sad people!
I don't mean this in a critical way but I'm sure every one has done it once in their lifetime though. Just seeing someone at the mall with really bad teeth or smelly hair would make you want to judge them. The real answers is that it's human nature.