He lets his guests sign his wall.
I'm not talking, like, a guestbook or a dry erase board. Davidson's guests write on his actual wall. If I did something like that in my room, my mother would have handed me that drywall for dinner. Also, one of the lights on his moon phase neon has burned out, and that somehow makes this all feel more real. Later, we see his friends getting tattoos in his totally clean and sanitary kitchenette.