I think I largely do it because of anxiety, whether directly or indirectly. Directly, when I'm anxious, it helps me regain focus in general, stops racing thoughts...I'm not sure how it does this, but it does. Indirectly, because I allow anxiety to dictate my actions sometimes in certain situations (usually running away from some sort of social interaction) I do it as punishment for allowing that to happen.

So I guess it's two because of two different reasons that boil down to a single, main reason.