Castiel was sick. Cas was sick and dying and there was nothing Dean could do about it. This was one problem Dean Winchester couldn't get through. That's what everybody told him. Still, he spent his days going through all of the bunker files looking for an answer. So far, he'd found none. Turns out the Men of Letters knew crap about angels.
Cas didn't know any of this. Dean had done his best to seem indifferent about the whole thing, even though his heart felt like it was about to explode ever since Metatron had outed Cas. So when Cas left the bunker every morning to o God knows where, Dean practically tore apart the bunker sifting through papers. There was a lot more in the bunker on narcissistic jerks than righteous dicks with wings.
Even without the mark of Cain Dean never slept much anyway, so he spent his nights looking for answers too. He'd been through it before. Losing Cas, thinking he was dead. Dean couldn't go through it again. Cas was his best friend, and maybe a little bit more .Over the years, Dean had seen him grow from a baby in a trench coat to the leader of an army. An army Cas had just given up for Dean, who had just accused him of murder.
Dean downed, his beer, ashamed. He wished he could still get drunk easily, but drinking copious amounts of alcohol from a young age only raised his alcohol tolerance.
He couldn't let Cas down. Not again. Not after the first Angel War, when Cas didn't trust him enough to ask for help. Not after purgatory, not after whatever he'd done to make Cas hide the angel tablet from him. Not after the angels all fell and Dean had done nothing but shut Cas out. Not this time.
Dean flipped through the pages of the large book in front of him, pretending to read even though his tears blurred his vision.
Dean was sick. Dean was sick and dying, in a way that was changing him into something Cas barely recognized. Castiel could feel it, a sticky darkness deep inside of Dean that was ruining him. Cas didn't know what to do. He had to find a way, though. Cas screwed up a lot, that was evident, but he refused to let him screw up with this. Not when it was so important.
So every day, Cas left early in the morning to find Cain, or at least somebody who knew how to get rid of his mark. Cas missed the old Dean. The dean that cared to much and wasn't cared for enough. The Dean that always tried to teach Cas what it means to be human, and never gave up no matter how far Cas strayed. He missed the Dean that loved Cas.
When Cas returned to the bunker, Dean was usually holed up inside his room, listening to music. Cas always knocked on the door, and when Dean didn't immediately answer, banged on it. Dean would swing open the door, angry and worried as usual. And when Cas said nothing, Dean's freckled face would always turn red with anger and shut the door on him. Cas would take the anger, though, for knowing Dean was alive and well. He had a fear that he would come home one day to find Dean dead, and was always relieved to see him alive and kicking. Then Cas would usually go see Sam or read a book, (Damn Metatron) and they would go through it again the next day.
Cas wished he still had wings, if only so he could bypass Deans locked door and check up on him thoroughly. Unfortunately, all Cas could do was search for answers. He couldn't let Dean die. Not his Dean. He'd been so pissed when he found the mark of Cain, but now Cas had wished he'd just held Dean tightly in his arms and told him it would all be okay.
Dean deserved better; he deserved to love himself, he deserved a functional family, and he deserved a home and rest. Cas couldn't give him that, but maybe he could give him a life and time for Dean to find all those things.
He couldn't let Dean down. Not again. Not after Cas lied to him and made a deal with Crowley. Not after he hurt Sam and abandoned Dean in purgatory. Not after he betrayed Dean, almost killing him. Dean may not trust Cas, but Cas was still determined to save the man that taught him to live, and love.