MAD confused. Like.. Seriously!?

i just moved to this new town and i met a guy im becoming really good friends with. he tells me im pretty nd i can tell him anything. he sends me hearts and we makeup cute nicknames for eachother and he seems to REAALLY like me. But he has a girlfriend. and whenever i ask how they are he says shes pretty and sweet and that he's lucky to have her. so theres no chance that theyre breaking up. why does he keep flirting with me if he's so happy to have his girlfriend ???