Dean says a prayer every night before he falls asleep.
This isn't because of religion, or any kind of faith he may or may not have. When he was little, Mary would sit on the side of his bed, take his hands in hers, and say a prayer.
Dear God, please keep Dean safe tonight and let the angels watch over him. Amen.
When she was pregnant, she would put Dean's hand on her belly and pray for little Sammy too. The night she died, she prayed for her sons' safety. Never her own.
So Dean keeps the tradition. He's never told anyone, but he does it. Sometimes he'll say a short prayer. Something like, "God, if you're up there, give me a break, and maybe a stripper or two."
Sometimes they're longer conversations. Conversations about Sam, John, hunting. Harsh, angry words, combined with pleading, softer ones.
He's never thought much about it. Praying is habitual to him and it keeps him sane. He's never considered his prayers actually being heard by someone.
But Castiel has been listening for the whole of Dean's life. And tonight, when Dean prays, he'll be there listening.
He listens because he cares. He listens because he knows Dean needs someone who does just that - listens - without giving him grief or judging him.
Castiel listens because he wants to understand Dean.
And he hopes that someday maybe, just maybe, one prayer will be for him.