Why would he do this to me?

so this guy gave me so many signals he liked me but honestly i never gave him any in return. now he has a girlfriend. do you think its possible that he did like me and he didnt know i liked him so he just moved on? and if thats the case then if they break up do you think he could go back to liking me? i really like him but was too shy to do anything and figured he would never like me so never took the things he did as signs but now lookingg back.