I agree with what most people have already stated, especially Ambivalent. I think that maybe some people lack empathy and they aren't even aware that they're being rude at times. Maybe their parents never taught them any manners. That can be used as a sarcastic way to let them know what an idiot they're being, maybe even as a joke by saying, didn't your mother ever teach you any manners? lol!

Sometimes some people are just stuck-up jerks who think that they're better than most people. If they somehow perceive you as being "beneath" them in social status, than it seems to me as if they think it's OK to treat you like crap. Other times when they see that their friends are being rude and getting away with it, I guess that they feel the need to do the same in order to fit in and be seen as "cool". It's really lame and sad.

Other times some people are just plain mean and sadistic and they get a sick thrill from seeing other people getting upset at their reactions. They are often angry and unhappy people. A lot of them are narcissists who lack empathy. It's all about them and when you don't kiss their butt, they hate that and end up lashing out at you when you don't do what they want when they want. They're just entitled brats.