i had read somewhere that it could be attributed to a cleaning response like something in its slime coat is irritating it and "likes" to be rubbed to remove the irritation. that being said it could also be a learned behavior, and the fish is comfortable with you and know not to bite the hand that feeds you.
this is why i love marine fish because of the personalities they have. people that say "it's just a fish" have obviously never had a marine tank. my old sailfin would eat from my fingers and let me touch her occasionally when she saw fit.