I look at it like this... We've all spoken to rude people over the phone, right? What is your first gut reaction? To be rude right back. That's how easily the Dark Side flows. You don't even have to think about it. It sometimes even feels good to be rude right back. Being able to control your anger and speak calmly to a rude person is much more difficult. It's that restraint that keeps you from the lure of the Dark Side, if you take my meaning. Of course, this is a very simplified comparison, but you get the gist.