Anger is a wasted emotion.
Too much energy, too much effort spent on a negative feeling that gets you no closer to being happy than the original catalyst that caused it. Many more worthwhile pursuits exist than to than to sit around seething in anger.
Anger at past happenings are better off being let go. The past cannot be changed, better to look toward the future. Anger at people can be distilled into two categories. People you care about and people you don't. The latter should simply be dismissed. Being mad at this person only serves to make you unhappy. It can never have a better resolution. Again just let it go. In the other case, it is much better to try and understand why the person did what was they did that made you mad. This is a much quicker path to resolution than fuming over what could be little more than a misunderstanding.
Living life, is about finding happiness. It really is just too short to waste precious time on being angry. There is too much to be done, and so little time to do it in.