Why do people have to be rude when they don't agree with your opinion/question/post? it's ok not to have the same opinion but what makes them be rude

They don't >> have << to be rude. They just seem to enjoy it! And really at some level they know if they acted like that in the real world, someone would punch them. I kind of enjoy it when a person has so little courage they need to pretend to be tough on the internet. It's like they shared a little secret with me about how insecure and unimportant they are.