You drew an analogy to pain as an unwanted gift: I think an even better analogy is with rage. On Steven Pinker's account, a hot-temper is a way to signal that you're unconditionally pre-committed to wreak havok if anyone harms you, even if, after having been harmed, it is no longer in your interest to do so. Temper is a signal that you have mind-control angel on your shoulder that sends you uncontrollably crazy when you are wronged, and is hence a deterrent to anyone that might harm you.