I think...without having their whole lives just tangled up in one another, their relationship would be....less, sort of, sweet and sad and wrong, and that would make me a sad panda. They don't know how to exist without one another because they never really have (I refuse to believe that Dean did not bug Sam at all for three years) and if they weren't brothers then there would have been a time when they were apart and NO. WRONG. STOP CREEPING ME OUT.