Perhaps we were brought up in a family where anger was unthinkable and never mentioned; voices were never raised. Perhaps everything was bottled up because we were afraid of anger. But we were angry!
It's hard to be angry appropriately. It needs to be learned, like so many things in our emotional life. If we haven't learned to direct our anger in appropriate ways, we may find ourselves flying into sudden, inexplicable and unfocused rages that scare us and people around us. Or else we behave sullenly and irritably for no apparent reason. Or, we get mad now for something that happened long ago, maybe even years ago.
As we try to better ourselves through personal growth, we learn how to direct our anger and get angry in a justifiable and appropriate way. It's good to get rid of our anger for the past, so that we can concentrate on living fully in the present.