As individuals we all deal with stress, difficult situations and people differently. How quick we are to anger or how easily we are provoked, can depend on a number of factors. One is personality; Inherited personality traits and environment. Another can be learned behaviour. For example, those people who come from family backgrounds in which anger is often demonstrated; will believe that this is normal. They may also see it as a very effective way of getting what they want.
Though I believe we all have our own inner moral compass which tells us when something is not right, there will be some immature or emotionally weak individuals who will use aggressive tactics to get what they want, without thought or care for another person. Rather then using gentle persuasion or debate.
Read More: Dealing With Anger