Anger is something that arises when you feel that you are being treated unfairly.
An easy example of this would be sitting in traffic. For instance, there’s a right-turn-only lane and somebody drives up that lane and merges over several cars in front of me.
Normally this would have pissed you off to no end, because “that’s not how you’re supposed to do it! Now all of us who did it correctly have to drive behind you!” and while that was true, it was really just us projecting our own jealousy of the car being farther ahead than we were.