Dean would sometimes space out, and Castiel knew why. His husband has always had the fear of losing his family and loved ones. His fears became true, more than a several times, really. But Cas told him to face his fears in his own head and overcome it, because he'd feel horrible and couldn't bear to see his husband so sad. He knew the first time Sam died Dean was a mess, he knew because he was the one who saved him from Hell when his husband sold his soul to bring his brother back.
Cas was lost in his thoughts as well and didn't realize Dean looking at him.
"What's wrong? You okay?"
"Yeah, I'm all right" answered Dean. But Castiel knew he was lying, Dean would always talk in that high tone when he lied about such small things.
"Seriously Dean, tell me. I know you are not okay at all."
Dean stared at him, not with puzzlement, but with great grief. "The first time- the first time you were in danger, the first time as a human, you died. I couldn't save you. But you did wake up and the first word I heard was "Dean". What was that? I mean, I did let you down."
Now, as Dean was looking at the very man whom he let down, he was astonished. There was that glimpse full of love in his eyes. His lips were curves, he was smiling. Dean did not understand what was going on, and as he opened his mouth to ask him what happened, his husband kissed him.
"When humans die, the good ones go to heaven. You know that, you've been there yourself. Remember the fireworks and your memories with you mom? Those were your best memories. So going to Heaven is just like dreaming, in a way: you see good things happen, and you see what you wish to see. The only difference is instead of creating a new scenario, you see the best things that have ever happened to you. And you know what my Heaven is? It is the one in which I have you. It was that time you took me out for drinks, it was the time you first looked at me, and it was the time I first saw you. That's my Heaven. My Heaven is the one I have with you. I was in Heaven with you and I just woke up. What you heard was the best thing that has ever happened to me. It was you."