The act of shipping slash couples was once considered taboo, but now has become commonplace. The gender pairing of couples is no longer limiting on what is deemed acceptable for the purpose of shipping. However, there are still lines that are being crossed in the name of shipping, and one of those is within incestuous pairings. Breaking free of the societal boundary of familial love in order to explore physical love might be frowned upon in reality, but it has not stopped fans from taking their favorite sibling characters and adding a new element to their relationship.
A noted representation of this act of familial shipping is in the Supernatural fandom, where Wincest became the prominent slash relationship that permeated the community for many years. The canon representation of two characters, who were as close as brothers because they were actual brothers, only served to fuel the exploration of slash potential among fans. Because these two characters had been kept separate from normal social interactions growing up, only able to depend on each other for many of their basic needs, it was not a far leap to contemplate that they had sought each other out for physical needs as well.
In time, there was canon acknowledgment of fandom's incestuous interest, which both noted the taboo nature of the pairing and also withheld a condemnation of such exploration. And so the fandom exploration continued. But what pushes fans to want to introduce a sexual aspect to family relationships in general? Is it that emotional exploration of two male characters does not fit into what society deems as platonic, and therefore must move into the realm of lovers? Perhaps it's the draw of being involved in a forbidden area of fandom, thereby making it more appealing? Or is it that the television writers are borrowing from a romantic storyline, and the components of such interactions are clearly seen through the brotherly interplay?
Maybe it's all of these reasons, or none. Perhaps it comes down to the fact that it's the Winchesters, and their situation naturally lends itself to this kind of relationship. For whatever reason, Dean and Sam Winchester as brothers has easily slipped into Dean and Sam as lovers. Incest might be a societal taboo, but Wincest no longer is.