Why would a guy change his behaviour towards a girl?

he used to be soooooo nice and kind.....we always used to mess around saying i love you too each other and now its been a few weeks he has started acting mean and i dont how come he has suddenly changed his behaviour towards me,,,,,he is soooo nice with everyone else, he only acts rude and mean with me.....today we were talking and he didnt answer my question so i said i think you have got a problem with ears n he said nooo i have got a problem with your face and i can spend whole night telling you whats wrong with it and then he was like ooo i am only joking and i love you reaallyyy........what the hell he is doing?what should i do? should i act mean tooooo? lots of people have noticed that he is being mean with me but why?