When you feel angry towards someone and want to say something unkind, that is all the more reason to speak kindly. If someone provokes you and you respond with anger, you are reinforcing anger as a part of your personality.
So returning kindness for unkindness is not simply being kind to that particular person. You're being kinder to yourself, because you are undoing a compulsion, taking one more step towards being free. You are turning wrath away from yourself, as well as being kind to the other person.
— Eknath Easwaran, Words To Live By