I usually go as short as I can go without razor bumps. I am transgender, so I usually write it off as dysphoria rearing its ugly head in. But the urges come whenever I am particularly stressed out or feeling negative about my self-image.
I find my short hair very comforting because I become my own comfort object. My hair is soft like a stuffed animal, and rubbing it releases oxytocin as well as just making myself feel cute and cuddly because of the texture.
It's funny -- I get my words mixed up frequently, so I might say to someone, "Yeah, I think I'm going to shave my head off this weekend!" Oops!