Dean couldn't remember the Cas from the past. When he was still Castiel. The Angel of the Lord, who intimidated him more than he would have ever admitted to anyone. He was even able to successfully hide it from himself for some time.
They both changed in a way neither of them would have expected or wished for.
Dean lost all capability of feeling good, all that was left was rage, exhaustion and guilt. It's what kept him going for the last five years in a lost world, that maybe won't be there tomorrow. It also prevents his attention to focus on the heartache that would be burning in his chest every time he looks at Castiel.
The man lost everything, he fell, he rebelled, he abandoned everything he believed in. Now he is a broken shell without a purpose in a world that's not worth living for anymore. Dean realized long ago that if it wasn't for him, Cas would have left this earth already, one way or another.
The implicitness of their relationship and the way he has taken the other man's loyalty for granted for so many years now should terrify Dean. It would have crushed him in the past, but not now. Instead, he takes advantage of Castiel's blind devotion, wasting and breaking the other man even more.
Cas would follow his every command. He doesn't question him, he doesn't complain. He stopped doing that a long time ago, seeing that there was no way of knocking Dean off his adamant course to save Sam, sacrificing the earth on his way.
They spent countless nights together during the past years, trying to make them feel by clinging onto each other, bruising each other. A desperate attempt to bring back emotions long gone and to ignore, if only for some hours, the inevitable fact that the world is coming to an end.
Castiel is content with every comfort he can get out of their heated and desperate encounters. He never asked for more. Dean needs those occasional nights to remind himself that there is still some human part inside of him, behind the veil of feeling angry, helpless and lost.
And Castiel offers everything he has left, he always did, right from the start. His devotion for the other man was strong from the beginning, scratching the border of self-abandonment. Somewhere along the way he started to feel drawn to Dean in ways he had never felt before. He made it his purpose to safe this man from his doubts and self-loathing, giving him a reason to trust in anyone else but himself again.
He lost this fight. He had to watch Dean fall apart, starting with the loss of his brother's support and ending with Sam's surrender to Lucifer. He failed in keeping Dean from suffering. He was the one who raised him from hell, made him obey to his role in this whole game and he failed, letting Dean lose all hope to ever achieve happiness in his life.
Castiel sees this as his punishment. Every night they spent together he has to witness what Dean has become, feel his anger and rage through every pore, raining down on him in an attempt to unload some of his desperation onto Cas' shoulders.
And Castiel will accept it. It's what he deserves, after all.