Road rage.
I used to be pretty bad about it, but now I'm not. I ask myself if the other driver is making a conscious decision to be a dick, or if he's just not paying attention or whatever.
If you want to go 10 below the speed limit, I won't ride your ass because sure, we can go 10 below. I don't care. I leave myself plenty of time to get somewhere anyway and I don't have that "I need to be there yesterday" mentality anymore. If you need to change lanes and get in front of me, I'll slow down a little. Just use your damn blinker so I know WTF is going on and be reasonable about it--don't get in literally inches in front of my bumper and make me slam on the brakes.
I really don't understand why other people consider lane changes as a slight. Sometimes your exit is coming up and you have to get over, and you didn't or couldn't sooner. Why would someone take that personally? I don't get it. I really don't see the big deal of someone pulling in front of me and possibly making me slow down a little... Unless, of course, you pull in so close that I fear a collision will happen and I have to slam on my brakes. Then I get pretty pissed.
My husband, however, sees red whenever anyone "cuts him off", which he considers getting in below 30 yards from the front of his bumper or making him slow down just a bit. I don't think I've been "cut off" unless I have to brake sharply and/or take evasive action to avoid a collision. (Even then I tend to get over it pretty quickly.)
What really gets my blood boiling is people riding my ass when I'm already going like 10 over. Especially when I'm keeping up with the person in front of me. Why are you mad at me about the speed we're going? Take it up with the person in front. Jesus f*cking Christ.