Well, there are many reasons, but they all boil down to his being a lousy person, really.
Yeah, maybe he cheated and feels guilty. Or maybe he's a jerk. Or maybe he's excessively paranoid and doesn't trust you. Or maybe he's latching onto your nonexistent cheating as an excuse to dump you. Point is, he obviously doesn't want to be with you...so why worry about him?
Let him go be paranoid about someone else, and find someone who trusts and respects you. You'll be a lot better off.
Who want's a dude like that?!? Next him...
Man, the amount of people I know, both male and female, who have encountered this issue in relationships truly astounds me. There seems to be a huge lack of trust in relationships these days. Everyone needs to please calm down. I have never cheated in my life but have had four ex girlfriends leave me for 'possibly' cheating.
Just leave the person alone, they are insecure and 100% not feeling guilty. He will convince himself beyond a doubt that you cheated, maybe in a year or so he'll realize he is being childish but for now I doubt it. But for now he will say to himself "that time she was 20min late she must have been with another man! She smelled like the interior of a Civic, I drive an Altima!" Or some insane crap like that.
Move on, find someone who you can be yourself with and who will trust you.
Same situation here