Can't add much to RichardMurray's comment. Bad upbringing and/or immaturity.
If you're talking about rudeness on the Internet it often comes from people who have the courage to be rude to strangers they don't have to face, but who would be too cowardly to be rude to you face-to-face.
It doesn't seem so bad when you treat them like the cretins they are.
it depends on you definition of rude .... like fogeting to say please and thankyou .. or being insulting and offensive rude .... its all a matter of respect .. if someone doesnt have respect for themselfs or others around them then that is were rudeness begins.... i dont care what background you come from if you respect someone you will never intentionally be rude to them
People are rude when they have failed to understand that life is a group project. You can't do much alone, you don't know what you are going to need, or from whom. So, apart from being mature, it just makes good sense not to be rude. However, on Ask and places, it's ok to be a little less polite because it's better to be terse!
You don't get to the nub of arguments by being nice to everybody!