Why was Dean acting like an ass to Cas in season 6?
Don’t worry about it, though. We’ve all been there, and especially me.
So, I won’t get into this a lot because season 6 has been discussed so much - some meta bloggers, like @elizabethrobertajones, even have weirdly specific tags for it (hers is ‘we don’t talk about season six’, which I always assumed was a veiled threat and, as it turns out, she thinks it was a very romantic season and we don’t discuss it nearly enough).
There are various theories about how this season was built, and one of them is that it was supposed to turn Cas for good - to make him into an enemy and then eliminate him from the show, if I remember correctly, so the general consensus seems to be, Why wasn’t Dean more of an ass to Cas in season 6? This was a narrative centered on misunderstanding and miscommunication, and from Dean’s point of view, Cas was acting like a demented Callahan type for no reason, which, given angels were (they still are, but back then it was particularly noticeable) the most powerful creatures Dean’d ever encountered, was incredibly dangerous, not to mention unpredictable. Dean should have wanted to take Cas out just to be on the safe side, and if it had been anyone else, he would have done it. But, of course, deep bond and stuff. Even after Cas’ done the unforgivable and hurt Sam, possibly for good (protect Sam: remember that’s Dean’s genetic imprinting, and he steamrolls over both friends and enemies to get that done), Dean still has enough empathy and affection for Cas to come clean about his own feelings, and to try and help Cas, or even save him, if he can. That, I think, is unprecedented?
Something that doesn’t come up a lot as a reason why Dean was so awful to Cas during this season (and therefore, what I’ll focus on here) is how Dean constantly refuses to see Cas for what he is - not a human being, but an unknowable, alien, otherwordly creature.
Now, from Dean’s perspective (at the beginning of season 4), angels are not monsters, or things he hunts, or things that exist in the real world; they are, instead much more close and personal than that. They are a cherished memory of his mother, and they are, therefore, an emotional concept which symbolizes peace and being safe and thinking that things could, one day, be alright. This is thrown into particular sharp contrast if we compare Dean’s religious beliefs to Sam’s - we know that Dean doesn’t believe in God, and therefore angels, and that he doesn’t pray. So, for him, angels really are this intimate, childish thing he’s allowed himself to cling to all these years: his mother’s voice, full of love, biding him goodnight. And when Cas shows up, it’s painfully clear that Dean takes his very existence personally, and he’s not at all happy with any part of it. Cas is important in the narrative because he sort of ‘pushes’ Dean out of his comfort zone; he challenges him, and makes him feel out of control in a life where Dean’s fought so hard to be in control at all times (because someone had to be). In a way, I wouldn’t be surprised if Cas’ overt sexual aggressiveness was planned for exactly this reason - because Dean’s been written as bi from the start, and yet this is a part of himself he keep a tight rein over, and Cas’ behaviour very nearly shatters all that. We’ve seen Dean’s uncomfortable with being flirted at, and he’s uncomfortable with anyone being too close to him (in every sense) and Cas, in this sense, is a nuclear reaction. All those secrets Dean’s fought so hard to protect from his brother and Bobby and everyone else - now there’s someone who knows them. All of them, including what he really thinks about himself and the shameful things he did in Hell and how they made him feel. And the fact Cas was always in Dean’s personal space was partly meant, I think, to symbolize this intrusion into Dean’s mind and soul.
(It must have been terrifying, really.)
And the thing is, out of all the possible responses Dean could have to this gobsmacking, life-changing revelation (that God exists and angels exist and one of them saved him from Hell and is now following him around), what Dean chooses to do is extremely revealing: he starts treating Cas like a human.