Chromosomes!

X: Okay, so what if I don't want to give up on her?
Y: You don't call.
X: But you said I don't call if I wanted to give up on her.
Y: Right.
X: So I don't call either way?
Y: Right.
X: So what's the difference?
Y: There is no difference right now. See, X, the only difference between giving up and not giving up is if you take her back when she wants to come back. But you can't do anything to make her want to come back. In fact, you can only do stuff to make her not want to come back.
X: So the only difference is if I forget about her or just pretend to forget about her?
Y: Right.
X: Well that sucks.
Y: Yeah, it sucks.
X: So it's just like a retroactive decision, then? I mean I could, like, forget about her and then when she comes back make like I just pretended to forget about her?
Y: Right. Although probably more likely the opposite.
X: What do you mean?
Y: I mean at first you're going to pretend to forget about her, you'll not call her, I don't know, whatever... but then eventually, you really will forget about her.
X: Well what if she comes back first?
Y: Mmmm... see, that's the thing,somehow they know not to come back until you really forget.
X: There's the rub.
Y: There's the rub.

No one is coming back because there is nothing to come back to.