"Don't ever change."
He grasped the angel's shoulder in much the way Castiel must have gripped his own saving him, as though he might disappear if he let go, but he did let go almost immediately and took a step back. His friend was still there, smug for having saved Dean once again.
They moved on.
Seeing the future shook Dean more than he thought it might. Days later, he realized that, staring up at the ceiling of some dirty motel room. Seeing Sam possessed by Lucifer hurt like hell, worse than Hell, became a call to action. He felt, despite all the higher forces in the world trying to tell him otherwise, he could stop that future, because he understood how to keep his little brother from slipping through the cracks. And as long as they were looking out for one another, as brothers, he felt instinctively that Sam would keep him from turning into what the heartless son of a bitch he had seen in the future.
Castiel proved a different matter altogether.
In 2014, Dean had truly seen the apocalypse in his friend, the way the End would fundamentally change everyone going through it. At the time, the changes in Castiel had been bewildering, slightly frightening, with just an undercurrent of twisted humor in the humanity the angel had taken on. While he had been there, actually in the future, other matters had been more pressing, of course, more immediate, and Dean hadn't given much thought to the drastic changes in Cas except to accept them as some alternate future he could change.
But could he really?
The bitter smile haunted him, the hopeless, hollow abandon in his eyes. The hazy sex-addict he had met there was not the Cas he knew, nor should it ever be. How could he possibly stop Castiel going off the deep end if Heaven failed him? or if he, for some other reason, lost his wings permanently? Dean didn't know much about angelic anatomy, but he wasn't sure that they had souls in the way that humans do. And yet seeing Castiel there had been like seeing the end stage apocalypse of a soul.
Spiraling deterioration. Desperate scrambling to forget what it's like to feel.
Dean knew how that felt, knew the look of a broken man, because he had seen it in the mirror. His stomach churned as he stared blindly at the ceiling, trying to forget the angel's potential future. Thinking about all that Cas did, how the angel had slowly changed since they had met. He realized that it couldn't have been the end of days or even losing his juice that had caused Castiel to fall. It had been himself.
For whatever ungodly reason, the angel held him in high regard. He sacrificed himself for Dean, bent over backward to keep his trust, and for no reason that Dean could see. They were friends, better friends than Dean could ever quite put into words. He had seen himself in the future, the hopeless bastard he had become, someone who didn't even care about his own brother. No wonder Castiel had lost hope as well.
The drug-addled grin sliced its way lop-sided across his mind, but he pushed it away quickly, telling himself that it wasn't real, just a possibility, and one he wouldn't let happen. Instead, he focused on the smugness he had seen most recently, the optimism. The apocalypse was still imminent; it was all around them, but he hadn't yet seen the End in his friend's eyes.
And he hoped to God he never would.