Well, not having read a single SPN fanfic (of either the incest or the non-incest kind - I did read two or so crossovers with other fandoms, but there was no sex of either kind involved), I couldn't comment on SPN fannish motivation, but:

why do so many people want to read such extreme angst all the time?

Has she ever read BTVS and AtS fanfic? Okay, granted, that has the entire spectrum, and there is fluff as well, but you know what I mean.