He used to think he understood humans and then he met Dean Winchester. And everything he thought he knew ended up being wrong. He had seen a million men like Dean, a million lost souls just trying to find peace. But Castiel never met someone who so actively denied rest. Dean made him wonder if he really understood people at all.
After he pulled him from hell things changed. And they kept changing for all of them. And after everything Castiel had to admit he had never seen anyone like Dean before. Time and time again Castiel watched and wondered how such a person could exist. How someone with such great pain could walk around like he was numb. He watched Dean all the time. Heard him all the time. At first he just wanted to understand, to try and make sense of at least this one thing. It didn't take long for it to become something else. Dean became like a missing limb and Cas could always feel him there, an itch he couldn't scratch. All that pain and self-torment became constant static in his brain. And when it comes down to Dean or what he's been told he has to do, he knows the answer. Even though he is pretty sure it's the wrong one.
"Of course." He replied when Dean comes to him. There is no more than that to say. He will lay down his life for this silly mortal boy, of course. It will always be that way, even though he doesn't know it yet. Countless others have answered the same. Countless lives have lost in unfailing dedication to the Winchesters.
He had always seen Dean Winchester. He knows this now. When he looks back now, there was always a steady heartbeat in his ear that he could never quite place. Until he dove down into the pit to rescue some boy, some mysteriously important soul, and he finally found the source. It was all fire and pain, exactly how you would imagine Hell to be. The putrid scent of demons was almost too much and then there he was. Green in a sea of red, good in a burning valley of evil, there was Dean Winchester. All of the sudden Castiel could place the heartbeat. A thousand years with an unknown thumping in his head and as soon as he griped this boy's shoulder it all made sense.
Dean Winchester, an angel's destiny. And after, when the soul had been placed back in the body and Castiel was once again a force too big for the world he kept trying to reach out. To tell Dean that he was here, finally they had met.
But it was Dean, and he hadn't known then what that meant. That it would be years and years filled with more pain than he had felt in all the millennia he had been alive. That they would both fall short of each other's expectations. They would hurt, die and betray each other, over and over again. It might have been another thousand years since they had met. It sometimes felt that way. But Castiel has always been bad with time.
The boys look different now. Like old men, like they have been alive as long as he had. Still when he's around they put their ghost smiles and call him "Cas." As if the world wasn't constantly on the verge of ending. Pretending that none of them are responsible. That none of their names might soon be added to the list of people they have lost. To be honest though, Cas and the boys have been on that list a few times already.
So whenever Dean asks, will you come with me and probably die? When Dean begs for his life, broken and covered in blood. When Dean asks him to stop and put the world on hold. When Dean says to trust him. When Dean asks anything, really. Cas says the only thing he can anymore. It's not always the exact phrase, not always the precise words. But it's always the same in feeling.
"Of course."
And one day when the time comes, when it's the end and all is lost. When they look at each other and Dean asks him the question they've been avoiding for forever. Cas will dutifully reply:
"Of course, of course I love you."