I think it's possible it's not just for fun. Self harm is often a coping mechanism. You said you used it in response to a stressor before, but boredom or apathy can stem from stressors as well, and even if you feel calm when you're self harming, it's still doing something for you (like releasing stress or serving as an unhealthy grounding mechanism). People normally self harm for the same reason but it can evolve over the time, especially if it turns into a coping mechanism.

Some people do self harm for other reasons. I dissociate before self harming and spent a lot of time with my therapist figuring out what motivated my SI when the urge is there like all the time anyway. We still don't know. If it's not a coping mechanism for stress, it could be a way to punish yourself too. It can also be related to your relationships with other people, like wanting to scare or punish them or gather sympathy. Entertainment seems like an oversimplification of your motivation, this seems like something a therapist could really help you figure it. Sometimes, you don't need to fully understand the why in order to stop, though. Good luck!