The break up analogy would have worked better... (Spoilers)
If Zack had been the one to break up with Beth before she died. The fact that she broke up with him, then dies, then he remains obsessed, then she returns and returns his obsession, then he wants her to remain dead makes the entire thing confused.
The director states that the entire thing is an analogy for a break up, forgetting the things you dislike about your ex and only remembering the good, trying to get back together and remembering that theres a reason to let those things die. This makes more sense if Zack has rose colored glasses about his relationship with Beth once she has died, forgetting the reasons they broke up to begind with, then remembering once shes back as a zombie.