Author's Note: This was based off a post made by tumblr user noangelsinthegarrison, at their blog post/55824728925. You don't have to go look at the post if you don;t want to, but I suggest it. It was requested by user castielwithwalkers. Zapped it out really fast, apologies for any errors.
"Hey guys, I found it: why it had to be Cas! The third ingredient!"
All three of them look up when Kevin bursts back into the room. Dean zeroes in on Kevin like a hawk, but Sam glances back and forth between Kevin and the angel— well, fallen angel— in question. That's why he's the only one who see it when Cas swallows hard and glances at the floor. Uh oh.
"What?" Dean snaps. Ever since he stopped Sam from finishing the third trial, Dean's been a mess. Sam is pretty sure it's a combination of relief that Sam's okay, guilt that Dean was too selfish to let his brother die for the good of the world, anger at Metatron (and a little at Cas too) for screwing up heaven again, and that frantic weariness that overcomes someone when everything is happening too fast for them.
"It's the Grace of an angel in love with a human." Kevin smiles triumphantly around at the three of them, and Sam thinks that he probably doesn't even know about the elephant in the room that he just shot in the head. The kid does have bigger things on his mind than Cas's romantic interests, after all. This revelation was just a scholarly breakthrough for him.
When Sam won't look at him, Dean starts turning red, and Cas glanced down and starts inching towards the door, Kevin's smile fades. "No, but, it all makes sense. For the first trial, he needed the child of a past union of angel and human. Then, the cherubim bow, the thing that transmitted the love in the first place. So logically, to tie those two together, the third thing would be the love that the cherub planted that led to the child." The poor kid still doesn't know, Sam realizes. He stands jerkily, startling everyone. Cas pauses in his subtle flight, and Dean, still blushing, turns to look at his brother.
"That's great, Kevin. Good job." Sam claps the prophet on the shoulder and injects some false cheer into his voice that even Kevin knows is off. "Hey, you know, you've been studying that book all day— why don't you take a break and get some food? You've gotta be starving."
"Oh." Kevin shifts uneasily, blinking the sleep deprivation out of his eyes long enough to take in the tension in the air around him. "Oh, yeah. Um, I'm gonna go do that. Yeah. Thanks, Sam." The teenager scurries from the room like it's about to explode. Sam kind of wishes he could follow, and from the look on his face so does Cas.
Slowly, Dean turns to the angel. "Is that true?" His face is blank, and his tone gives away nothing. Sam can't even get a read on him, which is saying something considering how long he's been reading Dean's face.
Cas freezes and slowly raises his eyes to Dean. The look he gives Dean is nothing new: it's the same soulful, mournful, half-apologetic look as always. Except Sam knows that now that his brother is looking he'll notice the other things. The devotion to Dean, the faith and trust in him that still hasn't been shaken even after all this, the faint awe, and most importantly the love.
Oh yeah, Dean sees the love, and he doesn't know how to respond to it. So, in classic Winchester style, he takes a step back, shaking his head. No words, just a general rejection. Whether he's rejecting Cas, or rejecting the idea of an angel falling for him of all people (Sam thinks that's the most likely one), or just rejecting everything right now because it's all finally too much for him and he needs a break, it doesn't matter. Denial is what he gives, and that's what Cas gets.
Before Sam can say anything— and really, what would he say?— Cas spins on his heel and is gone in a whirl of grimy trenchcoat.