Mean People Mean Kids

Can someone please tell me why people/kids are mean? I realize sometimes its unintentional but when it goes beyond that it really makes me mad!!!!!! It makes me even madder when my child is the one people/kids are being mean too. She's not perfect by any means but does not deserve for adults and their kids to be ugly. I am really trying not to go off on anyone but it's really starting to make me angrier than I need to be. I have tried to turn the other cheek and say things like "well you don't really have anything in common with..." but when it's so in your face rude it just really gets to me. OVER IT!!!!! I need to just let it go but it's hard when you have a child crying over it. Help??????????