Whats going on?

okk so i've been going out with this guy for 2 years, and he goes through time periods of acting like he's REALLY interested, and then he goes through time periods like he's not interested AT ALL. I'm running out of ideas...whenever I hug or kiss him he looks at me really blankly and looks very distracted, and whenever I play hard to get, he asks me why i'm not being affectionate. He still hugs and kisses me, but he doesn't do it like he used to.....i can just tell that he doesn't really mean it....do you know what i mean?? Sometimes it seems like he's doing it just to p time =[. He has always told me that he wants to marry me when we get out of school, but the other day i asked him if he was sure that he was going to ask me and he told me that he didn't know anymore!! It really hurt...i don't know maybe i'm overreacting....this just started happening a couple of weeks ago...what can i do to make him want me like he used to?