Castiel was usually the silent type.
Two types of silences followed him - when he was curious, and taking in his surroundings, and the other where he was mad. Really mad. Furious.
The problem with him being so silent, was that you wouldn't really remember when he was with you, so when that deep, gravely voice echoed around a room, you would be rather startled.
In a complete twist of the norm, a drunken Cas was a loud Cas. The angel could talk your ear off when under the influence of alcohol - and it didn't help that the short man was a light-weight. Plus, a talkative Castiel was an open Castiel. He would share things - things that Dean didn't particularly like talking about.
That's why he wasn't really surprised when the dark-haired man drunkenly told him that he was in love him.
"I know."
The next day - when the angel had had the chance to sober up - he would silently creep up to the angel, and whisper, quiet as a mouse, an "I love you" into his ear. A giant grin spread across the angel's face at that.
Plus, Dean never forgot when Cas was in the room with him from then on. The dark-haired man would stay near his side, letting the blond know of his presence, and Dean found that much more comforting than the sporadic appearances of his angel, arriving in a flutter of wings, and staying silent the whole time. At least, with the short man clinging to his hand, he wouldn't lose him, ever. And that was what was important. Keeping his angel safe.
Because he loved him. And we all know what Dean is like for the people he loves.