What Makes People Angry?

I believe people get angry because within each and everyone of us, we have stress levels of how much we can tolerate, and once it hits a certain level that we can no longer control, we let out all that is bottled up inside us.  There are many ranges of anger and also in how people react to them.  Also many different factors causing the anger to occur like our expectations, beliefs, our own thoughts, and even when someone does something which is disrespectful or disappoints us.