WHO: Sam Winchester + Castiel WHEN: Saturday eveningish WHERE: Cas and Dean's room WHAT: When Sam comes back, he stops by to check in on Dean, only to find that Cas is the only one there. (Again, season 9 finale spoilers as usual.)
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To say things were a little tense between Cas and Dean was an understatement. Dean was testy, angry, and the fact that he was a demon was only making that worse, and Cas was flippant and rude. Sam's theory about healing Dean with the power of love was sweet, but like all things sweet it was incredibly impractical.
Cas had learned that over the last five years. He'd been trying to keep Dean together with the power of love since Sam said yes in 2009, but things just didn't work out that way. The world was more complicated than that, and all they could really do was try and hold on in a universe that didn't give a shit about things like love.
So maybe Cas had given up before he began, maybe he'd gotten into arguments with Dean because it was easy, because he felt petty, because his heart was broken. He was used to fighting with Dean, used to bickering with him, used to being facetious and having Dean tell him to shut the hell up. To that extent it just felt like old times.
When Dean wanted to go out, Cas didn't stop him. Maybe he should have, maybe it was wrong to let a baby-demon out of his trap, but Cas had seen enough to feel like Dean could handle himself for a little while. He was a baby demon, but he was a big boy, and enough of Dean seemed to be in there that Cas was confident that Dean wasn't going to start chewing on innocent grandmothers for fun. He liked to think he knew a thing or two about handling demons and their moods. Meg hadn't been all that complicated.
With Dean gone, Cas opted to light up a joint (he'd found sources already) and make himself comfortable on the floor, indulging in relaxing asanas and trying to free his mind.