I think that it was more of a case of Buffy WANTING to be in love with him than actually being in love. Riley was the perfect boyfriend, handsome, attentive, eager to please, accepted by her friends, and, most importantly, HUMAN. She was probably told time and again what a great guy he was, and how perfectly they matched, what a fantastic couple they made (no, we didn't see so much of that onscreen, but the attitude was there, nonetheless. Riley even gave up his military career (supposedly) to be with her.
But, in the end, Riley wasn't despite Xander's endorsement, the "long haul guy." That's not saying that he didn't really want to be, because he certainly did. The problem, I think, was that he wanted to be a partner, not a sidekick, and he just wasn't good enough. It's highly doubtful that ANY human, no matter how well trained and dedicated, could ever be good enough. (That's why he and Sam hit it off so quickly. They COULD be partners, just about as closely matched as two people could be. And I said partners, not husband and wife. But that's a topic that doesn't apply here.)
Bottom line is, Buffy wanted a normal relationship, with a normal boyfriend. Riley was the closest thing that she could get in that category, and she tried really hard to convince her friends and herself that she was, in fact, in love with him. In the end, though, as someone else said, there just wasn't enough monster in him to make it work.
