You know, interestingly enough, it's possible that an individual may be unconsciously holding on to pain and anger in a misguided attempt to reconnect with the person who inflicted wounds and generated trauma during a developmental stage in which she's fixated. And this occurs in a hopeless effort to achieve a wrap and a sense of healing. Indeed, even though the regression and fixation traps the individual within the walls of intense distress, they at least bring him close to the scene of the crime, and the perpetrator(s). And being at least close equates to having a shot at resolution. Does any of this connect with you?
Though not as detailed, I want to at least mention the cognitive point of view regarding anger. The cognitivists would submit that anger is an incredibly powerful emotion grounded in a real or perceived event. They'd go on to say that anger's presence in our lives may be generated by any combination of genetics, life-experience, poor conflict-management skills, and learned behavior. And they'd probably suggest that most people who display anger blame others, and situations, for all of the hubbub; as opposed to taking responsibility for their misguided expectations. Indeed, if the events at hand don't jibe with their perception and expectation of the way things should be, boom, all hell breaks loose.