Why does he do this ?:(?

im totally in lovee with this guy that i've known since 1st class. i have always liked him but only recently hav we started flirting. he tells me that he likes me and then i ask him does he want anything to happen with us and he says not now but he really likes me. then wheneve theres a party on he hooks up with another girl , after he hooks up with the girl he texts me and flirts again. i dont know what to do . please help.