Anger has many forms, the most insidious of which is resentment. Resentment is like the plant fungus you never discover you have until you start looking at the underside of leaves. It starts innocently enough. You bite your tongue when someone is rude. You agree to another person's plan because you don't have a better one. Little by little, your glossed over feelings begin to accumulate. You might even be aware that they are piling up, but you convince yourself your feelings don't matter or you worry about the consequences of sharing your feelings.
Your feelings do matter and at the end of the day, if you don't express them you are the one who suffers.