Anyone can become angry--that is easy. But to be angry with the right
person, to the right degree, at the right time, for the right purpose, and in
the right way; this is not easy. 
The Mechanics of Anger
Anger is a primitive emotion that arises in the limbic portion of the brain—a 500-million-year-old structure we inherited from fish on our climb up the evolutionary ladder. Its function is to prepare its possessor for a life-or-death struggle.
The chief sources of anger are pain and fear—hardly surprising given anger’s emergence in an eat-or-be-eaten environment.
The wellspring of anger is an area in each of the brain’s two frontal lobes called the amygdala, after the Latin word for “almond”, which it resembles in shape and size. The amygdalae are not conscious, and possess no judgment. The physiological and (to some degree) psychological response they produce is instantaneous and aimed solely at self-preservation. 
The functioning of the amygdalae is easily illustrated. Imagine that you are walking along a dark, lonely street when someone suddenly steps from a doorway in front of you. You will experience an immediate “startle” response, your body will contract sharply, your heart will pound, and you may even emit an unintended gasp or cry. In the next instant, you realize that the other person is an older woman who acknowledges you with a friendly nod and proceeds to cross the street and walk away from you. The momentary bolt of fear quickly subsides as you realize there is no threat.
The startle response occurs before you assess the threat because information from the senses follows two routes—a short one to the amygdalae and a longer one through the sensory cortex, where the threat potential of the situation is determined.
Anger’s other trigger—pain—is even easier to envision. Ever hit your thumb with a hammer? The instantaneous rage is no less real for having no legitimate target.
Every sensory organ report—sight, sound, odor or touch—passes through the amygdalae en route to the brain’s higher centers.  They serve as an alarm center for threatening stimuli, producing a wide range of defensive responses:
Your body, in an instant, is primed for flight or fight. In the world in which anger evolved, fighting was an option to be exercised only when running away was impossible. Since the fight would almost certainly be to the death, all the physical stops were pulled out.
For a fish in an ancient ocean (or a modern one, for that matter) all-out aggression was a sound response to an inescapable threat.  In human society, primitive aggression—anger directed at causing harm to another—is rarely appropriate.
The best-known example of a human experiment in unrestrained aggression was the Viking berserker. Before going into battle, the berserkers deliberately worked themselves into a homicidal rage, probably with the aid of alcohol and, perhaps, hallucinogenic plants. They were, as a result, utterly fearless, superhumanly strong and able to continue fighting despite wounds so severe that they often died of them after their fury abated.
Needless to say, these human Juggernauts terrified their foes. Unfortunately, they terrified their friends as well, since they indiscriminately struck down everyone they encountered on the battlefield. Between battles, they were so violent and quarrelsome that the Vikings finally outlawed the cult that practiced the berserkergang(berserker way). 
Even in the most warlike society, unrestrained aggression is, ultimately, a liability. In modern society, virtually anything done in such a state would constitute a felony.
While we sometimes picture our modern day athletes as inheritors of the aggressive character of ancient warriors or animal predators, even the most violent sports are tightly constrained by rules designed to minimize the mayhem.
It will have occurred to the careful reader at this point to ask how we know what to fear and when to become angry. That decision is made in the cortex, the thinking, aware part of our brains, which receive the sensory information via the long route. The cortex processes a rough representation of the potentially threatening object, which is flashed back to the amygdala. Simultaneously, the cortex refines the concept of the possible threat and compares it with the contents of explicit memory of previous threats. Having decided that the threat is real or false, it signals the decision to the amygdala, which either maintains the fear state, switches to aggression or shuts down the emergency response. 
What that means in the context of anger management is that becoming angry is a decision! It may happen almost instantaneously, and at a level below your active awareness, but it is choice your brain makes. In other words, events don’t make you angry, you make you angry! Ancient and primitive though it is, anger is triggered in a part of the brain you actively control. So it is an option, not an inevitability.
 Aristotle, Nicomachean Ethics,p. 10, (H. Rackham, ed.). trans., available at http://www.perseus.tufts.edu/cgi-bin/ptext?doc=Perseus:text:1999.01.0054, accessed 11/16/2004
 Though the limbic system belongs to what is commonly called the “reptile brain,” it originated in fish, which predate the evolution of reptiles by tens of million of years.
 Amaral, j and Oliviera, J., Limbic System: The Center of Emotions,available at www.healing-arts.org/n-r-limbic.htm, accessed 11/15/2004
 McGill University, The Brain from Top to Bottom: The Amygdala and its Allies,available at www.thebrain.mcgill.ca/flash/index_i.html, accessed 11/15/2004
 Angry Feelings and Aggressive Behavior¸University of Cambridge Counselling Service, 2001, p.3, available at www.counselling.cam.ac.uk/anger.html, accessed 11/10/2004
 The description of the functioning of the amygdalae focuses on only one aspect of their activity. They respond to virtually all the basic survival stimuli, including the presence of food, sexual partners, potential rivals, young at risk, etc. See The Amygdala and Its Allies,p. 1
 Ward, C., The Viking Answer Lady,available at www.vikinganswerlady.com/berserke.htm, accessed 11/1/04
 The Amygdala and Its Allies, p. 2