Flow. You can't overthink grappling, or you'll sabotage yourself. You have to let your mind flow like a river.

When you are thrown or being choked you are unable to focus on anything else than the moment itself. Its extremely liberating and picking both of those activities were detromental for my health

I agree with the others that Flow is part of why grappling is relaxing.

But, the another reason is that it's an amazing stress reliever. After a good grappling session, you will feel incredibly relaxed and calm.