Why do guys always come back?

So long story short - I was dating this guy, but we were never exclusive. He was seeing other girls. Things didn’t end well because I wanted more. Fast forward to two years, I ran into him at a bar. He started contacting me; however, he told me he was seeing someone. They were supposed to go on a big first date and he told me that if he doesn’t fall in love that day, he wanted to give us another chance. I stayed with him for a weekend and he introduced me to his friends. Something he never did before. He told me he would like to get into a relationship within a year or so. He didn’t specify that he wanted it with me. Also, he wants to be celibate. I might be overthinking. But could he actually be serious this time?