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It's proof of weakness, insecurity, and no class. There is never a good reason to be rude. Ever. Take the high ride to show the distance between your level of class and their lack thereof. And let's not forget, it makes even an attractive person ugly.
This is absolutely true! Every time I hear someone talking bad about one of their "friends" I think of this. I find though that most people who find fault in others have the lowest self-esteem possible and are only pointing out their own short-comings.