Why is that a guy will leave a girl and come back multiple times, even if sex was never involved?

Hes broken up with me 3 times and come back aIve taken him back. This last time I left him and he ended up sleeping with someone else before I decided to ask him if he wanted to work things out. He had said no because he was talking to some other girl that he really liked I suppose. Well she used him and now he wants to talk to me again because he said I make him happy. we were on and off for 9 months and I never slept with him. I love him but he always misses me when Im gone....Im afraid he'll do it again but he told me hes not living the way he wants. he says its hard to have/go after something you want when u always fk it up. ( refering to him and i). how do u know if hes sincere this time? i feel like a rebound or something....he confuses me but i still love him unfortunately. he admitted to never accidentally texting me cuz he said i couldnt just be like oh i think about u all the time and thats why i texted u. idk what to think anymore but i always think about him. its been 2.5 months since we broke up.