There are all kinds of reasons, theories and scientific studies about why some people are like that. Such as they grew up in an abusive household, or have mental health issues, hormones, stuff that just goes on forever and is always changing. I once knew a guy who had been moved into the group home I used to live in, who had an explosive temper and little things would set him off, and I was told it was because of a head injury and he couldn't control it. I really did not feel safe around him and he was soon moved somewhere else. Of course I feel like the biggest hypocrite when I say that because before my diagnosis with Asperger's I supposedly only had outbursts because I was "behaviorally dysfunctional" and it was my mother's fault.