The stories we tell ourselves.
When the world was more dangerous, anger was useful in protecting yourself and the ones you love. Nowadays, with the constant threat of danger no longer looming over head, anger is mostly destructive. Anger throws you into an altered state that obstructs your judgement and causes you to make poor, rash decisions that you often later regret.
Because of this, you should limit your time spent being angry. You can't completely do away with anger but you can limit your exposure by reclaiming control of the stories you tell yourself. More times than not, you aren't angry because of a situation. You are angry because of the story you are telling yourself about the situation. Sometimes this story is accurate. Most times, it's not.
Controlling anger is about controlling the stories you tell yourself.