Although some people are capable of changing, most human beings are creatures of habit. How many times has he left you? If it was more than once, obviously he did not miss you that bad after the first time or he wouldn't have repeated the same "mistake". Maybe he keeps leaving with prospects of something new or in his opinion "better" and when he fails at this, he comes back to you. I do not know him, but I'd assume that he is using you. Move on.