http://oz.wikia.com/wiki/Nimmie_Amee

The retconned version is a bit more of a transhumanist story. Nick Chopper abandoned Nimmie Amee after his series of cursed injuries deprived him of his heart — construed here as the seat of the emotions. He was (some time later) fitted with a new heart; but it was a kind heart, not a loving heart, and so he didn't return to her.

Aside from the anatomic specifics, it's a problem of maintaining goals under self-modification!