Cas knows Dean. From the moment he kept the flames from reaching him and Sam when they were running out of the fire in Lawrence, he had always been there. Watching.
He knows exactly how many crinkles form on his eyes and how many teeth show up when he smiles . He has watched that smile more than once, as he left a pie for Dean on his birthdays, or when he did good at school, when John wasn't there to celebrate it with him.
He knows exactly which are Dean's favorite radio stations, the lyrics to every single one of the songs on the cassette tapes at the impala, and how his heart skips a beat and pleasure fills him whenever he's singing along to them, his beautiful, strong, low voice blasting through his red lips, the same ones he uses when he prays for him, his perfect fanfiction green eyes shinning, as the impala blasts down the roads, looking for adventure, just like it's owner.
He knows exactly how many freckles are spread across his cheeks, how many eyelashes cover his beautiful fanfiction green eyes, and how his breath relaxes, along with his hole body, when he falls asleep. Dean sometimes smiles, with the happy dreams Cas gives him every now and then, his pure soul shinning even brighter. Cas smiles too, then.
He knows the exact path tears trace when Dean cries, and how they make his now dark green eyes shine, as they roll down his cheeks, crossing through the freckles on his cheeks and wetting his face, like they always do when there is something wrong with Sam, or when he just can't take it anymore. He wishes he didn't, though.
Castiel has never cried. He never understood the need for it. Even when Dean died and went to hell, he didn't shed a tear. He knew that he was going to get Dean's soul back, that he was gonna hold the purest soul he had ever seen, feel the familiar warmth and life coming from it, and howl as loud as he could, so that every single angel in creation could hear him;
"Dean Winchester is saved".
As he set the soul back into the body that he healed by memory, he couldn't help but leave a single mark on it, so that all the angels knew who that human belonged to.
It wasn't until he parked his car at the bunker and forced himself to make his way to the front door, until it opened and Sam's sobs and tears were all over his shoulder, not really until he saw Dean's lifeless body on the bed, the eyes closed, their perfect fanfiction green color never to shine again, the eye crinkles that always showed up when he smiled never to be seen again, his strong, seductive voice forever sealed on those lifeless lips, never to pray to him again, the steady heartbeat as he dreamed, the soul full of life and hope that he claimed as his gone, that Cas understood.
He cries then, because he doesn't know what to do.