I saw the chemistry right from the very first episode! It's like they're made for each other. The wincest thing came later though, I initally saw them only as brothers. It's during 'Phantom Traveller' that the wincest bug bit me. Remember the scene where Dean just woke up, he was in bed with his tee and boxers and Sam came in with the coffee and doughnuts and he was just looking at Dean? That definitely felt very, very slashy to me. And the fics sealed it for me. Heh. Bad me :)