After the talk with Castiel and his line “I dragged you out of hell, I can throw you back in.”, Dean is terrified. He does everything the angels ask of him, he respects all orders or rules he would normally ignore, so much so that it affects his personal life (e.g. no illegal parking, no bar fights or even credit card scams etc.). Sam of course notices it but doesn’t know what to make of it. When Cas finally catches on, he feels super guilty. (I would prefer it, if it would be gen and not Destiel 😊).