Were they ever a good couple?
I think she was into getting an ugly funny pseudo intellectual loser (with a stereotypical jewish baggage on him), but once into the game, she didn't really enjoy playing it.
He is into her (because at least, come on, he's lucky to be with anybody just look at him, and she's out of his league) but maybe realizes he's not up to par, or that she's a bit boring? I don't know what she lacks for him but there is clear disappointment from him.
Probably he just got bored of being in a relationship after a bit of routine.
Anyway, it's a romantic story, I love this movie, but from start to finish their affair is not what a couple of lovers looks like, ever.