I was actually against Wincest at first. Incest pairings seemed to be everywhere at the time, and it was pissing me off, so I wanted just one set of brothers to be non-incestuous. I wound up choosing the wrong brothers to make a stand with, though! ;P
I first started being unable to deny Wincest any longer in season 2. I think the first big moment for me was the scene in Croatoan when Dean essentially says that he’s tired and he would rather die than go on without Sam.
By the time Dean sold his soul to save Sam in AHBL1, I was COMPLETELY CONVINCED that Dean was in love with Sam. (And Sam being in love with Dean is easy to accept given the facts up until that point, too.)
I mean, who does that? Who finds a voodoo person to sell your soul to bring your sibling back from the dead because you can’t live without him? Nobody besides Dean Winchester, that’s who. Most people would be upset but try to move on, or go and talk to a counselor or a therapist of some kind, or go down a destructive path of alcoholism and drug abuse, or even try to kill themselves if it was THAT BAD, but I’ve never heard of ANYONE EVER resorting to freaking voodoo to get a loved one back. And THAT is what makes Wincest so undeniably canon to me. :’D
So, yeah, Dean selling his soul to save Sam. The first part of AHBL1. That’s when I started finally shipping and loving Wincest! XD