1. He doesn't want to get hurt. This is some scorched earth policy shit. You can't break his heart if he breaks yours first and throws his phone in a river and runs away and builds a cabin in the woods with his bare hands and the cabin doesn't even have Wi-Fi so it's not like he can get email or anything. This is not a sane alternative to dealing with an impending breakup.
2. He'd rather be single. Take solace: If a guy has to have a long, hard think about whether to give up dating you or being able to leave beer bottles on the coffee table for two days, he probably wasn't boyfriend material anyway.
3. You're more into him than he's into you. It's no fault of your own, but this imbalance is just awkward for some guys. You should be with someone who thinks you're the jam anyway.
4. He doesn't see a future with you. Yes, guys do think about the future of a relationship. And if they don't see one, why waste your time and theirs?
5. He doesn't think you're sexually compatible. Some people just aren't vibin' on each other's junk. If it's not working on a physical level and he's not willing to try something different, that dude's weak.
6. He just wanted any relationship. It's possible that, even if he didn't mean to, you got used. It might have been a rebound relationship he needed to get over an ex, or he was just depressed or trying to fill a life void. It sucks, but it happens.
7. He likes you, but not that much. Sometimes relationships start out great and don't go anywhere. Maybe there was no spark in the beginning, but you got along and hoped things would get better, and they just never did. Sadly, you were the Greg Kinnear to his Meg Ryan.
8. He's not on your relationship wavelength. Maybe he wanted to get more serious and you just wanted a friends with benefits sitch, or vice versa. Either way, it's a classic case of wrong person, wrong time.
9. You're taking him for granted. One-sided relationships get old really fast. No one wants to date Queen Cersei.
10. You have different values. This one is the ultimate dealbreaker. It's really, really hard to reach a compromise when he finds out he wants kids and you don't.
11. It's just you. Listen, the odds are you're a perfectly fine human being, and even though you can't see it now, he bailed just because the relationship wasn't going to work. Maybe he finds you annoying or boring or clingy or any number of things. Some people are just Liz Lemon/Wesley Snipes levels of incompatible, but it takes distance to see it.