They've always walked on a ledge. They've never walked more than a metre away from each other. They've always followed the same path, leading the same way. They have never strayed from the ledge, never completely separated the other from each other. They can't do that.
The walk on a tight rope and if one walks off the balance gives away and sends the other crashing down. They need each other to stay on their feet, they need the protection the other gives and they need everything connected to the other.
Co-dependent doesn't quite cut it, but it gets close. So many words could describe their relationship to each other, but in the end there is not just one word, it's always more than a thousand to pick between. Their relationship is unhealthy, yes, it's obsessive almost, and it's almost psychotic when you think about it.
They have never talked about it. Never mention that both of them know how sick their relation is. How badly they need the other, and how mutual that need is. It isn't something they'd discuss. There is no way out of it, not that they'd want it to be, and there's no way they could survive being without the other.
Sam and Dean Winchester were made by God to be an exception to every rule of the world.
They were made for each other. They were made to walk on the tight rope together, to keep the other in balance and to somehow make that closeness work.