Buffy doesn't love him like that, and she doesn't want that long term. And he leaves because he doesn't want to settle.
Oh absolutely. I don't dispute that at all. But I think that he misunderstands what being a Slayer is... he focusses on the physical - lets himself get bit because he wants to understand what her deal with vampires is. But that's never what it was about - we see in S6 that Spike never once bites Buffy and she never wants him to. It's *partly* to do with the inner darkness that she has, but I don't think it had to have been such an obstacle. Mostly it's the fact that Riley would *always* be secondary to the mission - which for Buffy is such a fundamental truth that she doesn't even think about it anymore (she had to kill Angel).

At the end of the day, they break down because she's not just that into him. They should. And they both do some dumb/insentive things, Riley more than Buffy, but there's no real villain.
*nods* Spring Summers has an excellnt analysis of them here.

Also I have to recommend this drabble by st_salieri which is the best encapsulation of Riley I have ever read!