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This shouldn't be happening. Sam knows that and every time Cas shows up he knows he should say something. Every time Cas pushes him down onto a motel bed or up against a wall, every time Cas corners him in the back room of a bar or in the shower, he knows he should stop him.
Every time Cas kisses him, hard and determined and like he's the only thing in the world that matters, Sam knows he should push him away and tell Cas to go find Dean. Because there's something there, between his brother and the angel. Something strange and huge and unconsummated that makes tension spark between the two of them at the slightest look or touch.
When Sam lets himself think about it he's uncomfortably sure that Cas only comes to Sam because Sam is simple, easy. With Sam all Cas has to deal with is lust, not whatever messed up type of love he'd have to untangle to be with Dean.
Right now, flat on his back on a motel bed with Cas fucking into him and Dean in the next room, Sam bites down on his lip so hard that it bleeds in order to stay quiet because if he eventually mans up enough to push Cas away there has to somewhere to push him to.
But Sam can't. He can't give this up yet. He doesn't have the willpower right now to be the agent of his own loneliness even if it's the right thing. He doesn't even have the willpower to stay quiet.
Cas clamps a hand hard over his mouth to muffle Sam's moans and as Sam looks up into depthless blue eyes Sam's suddenly sure that Cas knows all this too.