Why does my boyfriend do this?

okay, so im 17 and i live with my 18 year old boyfriend and his family. (we're not allowed to sleep in the same room) but this morning i walked into his room and he was masturbating and watching . i dont care if he does these things. i know its natural and what not. but im curious as to why he feels the need to do this when we have a minimum of once a day. its not like i'm not satisfying his ual needs. does he find me unappealing or something? why does he do this?