Dean and Sam Winchester were the vessels of Michael and Lucifer; that was their destiny, always had been. And before he'd met them Castiel would've agreed with his family on that. But since he'd met them Cas had changed his mind. Time and time again he'd watched as the two brothers fought and died beside each other, for each other and he realised something; there was nothing past, present or future that one Winchester would put in front of the other. He'd watched as they beat Fate itself, stopped the apocalypse and battled against both Heaven and Hell, just to keep the other from harm. Two brothers, so full of hatred for themselves yet love for each other. Cas watched as they fought; he saw a relationship built solely on lies and mistrust as they constantly forsook each other, and yet no matter how many lies were told, how many bad decisions were made, one Winchester would always die for the other.
But that wasn't all he'd seen. In between hunts and near-death experiences, in the times when they weren't fending off angels and demons alike, there was another side to the brothers. Day long car journeys filled with too loud rock music and truck-stop burgers while they teased and poked each other for hours on end, telling meaningless stories from their past. Cas had seen them fight countless times over trivial little things like who used all the toothpaste or who got the biggest bed, not forgiving each other for weeks because they couldn't work out who ate the last slice of pizza. He'd seen them go into hundreds of run-down bars and flirt with random women they were never going to see again, the cheeky grin on one's face when he knew the other was going to get laid. And he'd seen how every Christmas and Birthday they insisted they didn't want anything, until the day came and they had a lap full of cheap tack wrapped in yesterday's newspaper from some nameless gas station that would mean nothing to anyone else, but because they knew each other so well it was perfect.
Because under all the teasing and fighting Cas had seen what the Winchesters were like when they weren't drunk or joking or angry. They were two brothers who were so broken and co-dependant that they couldn't be whole when they were apart, they knew each other better than they knew themselves and knew exactly how to comfort each other when they realised just how damaged they were, but at the end of it all there was an underlying peace between them because as long as they were together they were safe. In both of their minds, above all instincts they had to protect one another, that was how it always was and how it had to be.
That was why Castiel knew his brothers and sisters were wrong; because the Winchesters' destinies weren't to become the vessels of heaven and hell's wrath. They belonged right here, cruising down an empty road, eating greasy burgers and fries, blasting out awful music in a black '67 Chevy Impala. Together.