I just started working on a story where, after War Stories, Mal and Zoe really *do* sleep together, and Wash is so upset that he packs his bags and leaves, so it's the new pilot (or, as River insists on calling him, "Fred Shirt") who dies.

I was going to say that post-Serenity, it's really hard to write happy, fluffy, W/Z except that, and my actual answer to your question, is that, when you think of it, everybody is going to die sooner or later, and we all KNOW that, so it's just icing on the cake that Dean is going to be in Hell temporarily a little sooner than he expected to get there permanently.