Because it is a part of your biological structure.
Anger, as a matter of fact, is not a personality trait. It’s a mechanism in our brain that we’re all born with.
How we react to anger is our personality trait. But anger, itself, is not.
What is your first thought when you experience anger? Is it shame? Sadness? Remorse? Guilt? Emptiness?
We all relate differently to anger, but the artefact of how to relate, I believe is a lot to do with our societal and familial expectation and lies heavily on what we were taught as children.
Anger in many cultures, is not considered a positive emotion, or at least not a welcome emotion. But the problem is this, anger inherently is not a negative or a bad thing, it’s actually our ally, a teacher who signals us that something isn’t quite right.
The reason why anger is unwelcome, is because how we react to anger. We’ve all been there, or if not have seen in movies, throwing tantrums (or objects), yelling, using negative words etc etc. Imagine if we lived in a world where when one experiences anger, we all go back to taking deep breaths, followed by a quiet moment to reflect and then discussing what triggered anger in a compassionate and rational manner - I highly doubt that anger will be labelled as a negative emotion.
So going back to
Make anger your ally, and listen to that emotion. It’s trying to tell you something, and take your time in digesting that emotion.
You might have to sit inward in discomfort, but don’t let your emotion go to waste, endure that discomfort (without throwing tantrums) and come out the other side - and make your anger your friend.