Why would a guy who rejected you, who was very eager to be your friend suddenly ignore you?

I liked this guy, he kinda rejected me adn afterwards he was treating me really as a friend, commenting on my fb page, starting convos online etc and then all of a sudden i dont see him for a few weeks dont hear a word from him and then the next time i see him he seems to be ignoring me and then when i talk to him on fb chat he didnt respond, he always responds! Help!!!!!!!!