Mm. Honestly, I think part of it has to do with end of season two and beginning of season three. But that ventures into spoiler-land! So, I'll just wait until you've seen it all.

Also, I dunno if this has anything to do with it, but I've always thought that where a person comes into the series has an impact on their topping-bottom mentality (and characterization in general). There are folks who got into the most recent episode and work backwards; there are people who start chronologically at the beginning; there are those who just skip randomly!

And this sounds dumbest of all, but I think reading fanfiction beforehand has the biggest effect. I mean, that's how a lot of people get into it-- by reading stuff ahead of time, and deciding to get into the show that way, and a lot of the better stuff was Sam-top and Dean-bottom, which just sort of built upon itself.

And then I have all these vague theories on the appeal of incest to people-- whether you like it because of the dirtybadwrong feel or in spite of it, and that's how preferences for top-bottom develop. And theories about how favorite characters tend to get put on the bottom, or whatever, but I need to get back to work!