Help!!please..................…

okay i use to like this guy for a while and then after a while it looked as if he started to like me we would go to this class for a coaching thingy and you could go as an option and we would always go together and as usual he would tease me and he ALWAYS would stare at me and we would always sit together but then i found out he was madly in love with this other girl even composed a song on youtube for her and we would still talk and all but then we moved groups and we stopped talking and or friendship faded away i wanna know why you think he did this