I was talking to someone on my friendslist a while back, who said that she'd read a selection of Sam/Dean incest fics out of curiosity to see why people shipped them, and been surprised by how despairing and grim most of the stories were. She'd got the impression that they were all happy and fluffy and "sibling sex is hot", and ended up even more puzzled than she had been in a "why do so many people want to read such extreme angst all the time?" way.