I have a guess on this. I only ever self injured to punish myself or to relieve anxiety or anger. Since self injury releases endorphins in your brain when you're feeling bad, then it makes people feel relief in ways, but if you're already happy, maybe you are doing it to stay out of the sorrow..to keep yourself stable, because since sibcalms people, and affects your mood, maybe you're relying on it to stabilize all your moods that you aren't sure how to deal with.
Maybe if you allowed yourself to be happy and say to yourself that you're going to go for a walk or clean instead then it will keep your mood stable as well probably and give you something productive to do.