Why are we so rude to one another online? Simple.
This is why, explained simply.
To give a more thorough explanation: It's far less likely that being a complete muppet online will lead you to get your face smashed in than what being a complete muppet in real life will. Being banned from a forum, or unfriended on Facebook, is about the extent of the punishment involved and, to be honest, isn't much of a punishment anyway. True, there is a bit of social stigma that can result in being unfriended on Facebook (it's the perfect platform if you want to badmouth someone or plant rumors about someone), but that's fairly normal in real life as well. I'm pretty sure that even the worst troll on these boards are the nicest people when you meet them in real life. Why? Again: Lack of any physical consequence.
If computers were equipped with hardware that kicked you in the groin every time you were an idiot online, you'd see far less idiots.