Why did he trade me for her when he said he like me?

when me and my x met again he was very touching with other grls. when we got a chance 2 talk he told me that he really likes me,and that he could open himself to me unlike the other grls.When he was saying these things he was getting teary eyed and he said it was because it was coming from his heart. 2days later...he began to sit wit another grl and then i caught him going to her room..i dont understand y he did it..when he was so sincere about it...he was hitting on me for a long time and was different with the other grls. and i dont know y he did that..when he was all over me