Trust

So this guy and I dated about two years ago. Never exclusive, so I knew he dated other girls too but we never talked about it. Me, on the other hand, never dated anyone else during that time; however, when we were on our “off” stage, I did see someone else. And he knew about it. Then I’d go back to him. I was always honest with him and he knew everything. But never once did I question him and the other girls. We ended things. Fast forward to now, we ran into each other. He wanted to see me. I wasn’t so quick to see him. Finally we hung out. Pick up from where we ended. I brought up the “what are we” question. Basically, he told me that he enjoys being around me. That he has so much fun. But when it comes to anything more than that. He can’t be with me because he doesn’t trust me. When I asked why, he said he just doesn’t trust me and wouldn’t explain. I don’t think I’ve ever given him a reason not to. Why does he even bother to be with me again if he knows it is never going to be a relationship? Why wouldn’t he trust me?