It may be worth it to seek therapy to try to deduce an answer as I'm sure it's impeding your life to a degree. Unfortunately, strangers on the internet know nothing about your past to piece things together.
I would guess you get mad because you can. If there was a social penalty to pay, and if the penalty mattered to you enough, you would probably not get mad, or would have learned to control your anger. If you are worried about this as a character trait, please see a counselor, as quiet recommends. There are a lot of good anger-management techniques, but you gotta wanna.
Few thoughts for you to consider, during the process of anger: generally speaking there are aspects of resentment or blame or judgement or guilt that are involved. These are all factors that may well contribute to the anger itself. There is sometimes a belief and an expectation when other people are involved.
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