The first rule of working the streets is not to get attached to any of the clients, but Sammy can't deny he's rather fond of John, who actually seems to be a decent guy, on top of being hot. The first rule of paying for sex is not to get attached to the prostitutes, but John is willing to admit to himself that he wishes Sammy was more than the guy he occasionally pays for sex.  It's a good thing rules are meant to be broken.