People get angry for all sorts of reasons, from the trivial ones (someone cut me off on the highway) to the really serious ones (people keep dying in Syria and nobody is doing anything about it). But, mostly, anger arises for trivial reasons. That's why the American Psychological Association has a section of its website devoted to anger management. Interestingly, it reads very much like one of the oldest treatises on the subject, On Anger, written by the Stoic philosopher Lucius Annaeus Seneca back in the first century CE.