Castiel keeps his eyes locked onto Sam's as the man's cock pushes all the way in, down his throat, and even as tears prick his eyes from taking him so deep, he makes a content, happy sound. This is where Sam belongs, and Castiel is grateful to receive him like this, the heavy weight of him inside his mouth, taking pleasure inside of Castiel. And he's glad to provide it.

Sam belongs inside of Castiel, claiming every part of him, filling him up and leaving his mark everywhere he can. So Castiel makes soft sounds. Moans and sighs so show his appreciation, to show how eager he is for more, for everything.