Why do I get mad so easily?

The smallest things set me off. I almost hate driving because I get so irritated. I dont turn into a psycho with crazy road rage, but I do find myself screaming a lot at the others cars. My friends have noticed my quick-to-anger attitude since they've known me. They used to do things to purposely make me mad because they found humor in my anger. I dont walk around mad all the time or anything like that, and actually Im probably one of the easiest people to get along with, because even if Im mad or dont like you, Im still nice. I dont go around being rude to sales people or servers or anything like that. Im actually fairly patient when it comes to people that are doing their jobs. If its a mistake then its a mistake and I can accept that, but there are some things that just set me off and I feel like im going to literally explode. If people could spontaneously combust I definitely would have by now. Ive thought about medication but I dont want to have any of the side effects that might come along with it. I really want one of those punching bags that are in the shape of a man, I really think wailing away on that thing for a few would make me feel much better. And thats another issue is that my anger leads me to want to hit things, I have scars for life on my knuckles from punching countless walls, and I throw stuff. It sounds really childish and stupid I know but its better than actually hitting a person (which is what I really want to do). My boyfriend tells me AT LEAST once a day to calm down, and I know Im over-reacting, but I dont know how to change?????