I told myself I wouldn't meta this episode....
So a lot of people don't like last night's episode. And there are reasons for that I understand. But here's one I don't.
The episode was poorly written because Dean was lenient with Cas on things he would not or could not forgive Sam for (deals with a demon, secrets, use of ethically questionable magical power sources). Dean being harder on Sam than Cas is OOC, or a betrayal of the brotherly relationship.
The argument above, my friends, is sort of bullshit. I propose three other possible explanations for Dean's changed behaviour.
1) Sam is his brother, and Dean has different expectations of his blood. Sam and Dean have always been a team, and Dean takes a betrayal from someone who is essentially a part of himself much more personally than one from Cas, even if he considers Cas a good friend. Dean feels a greater responsibility for Sam's choices, so not only does he blame Sam, but he also blames himself. Twice the pain = twice the anger.
2) Circumstances have changed, and Dean has a new appreciation for friends and family. Dean spent over a year without Sam or Cas or Bobby. He spent it with Lisa and Ben, who gave him a taste for a stable familial structure. Then he had to give that up, and got some shitty imitation of his brother in exchange. Oh, and some douchebag who calls himself their grandfather but tries to kill them both. Dean eventually got his Sam back, but that's unstable, temporary, fleeting since that wall could could come crashing down at any moment. Then he watched Rufus die, and Bobby grieve. See how Dean might be more than a little desperate to hang on to the closest things he's got to family? That's why he made the blank slank slate speech at Rufus' funeral, and why he doesn't want to believe the worst about Cas, and gives him every opportunity to turn back to the light. He can't afford to lose anyone else.
3) Most importantly, Dean has learned from the mistakes he made with Sam, and he doesn't want to repeat them. Dean was wrong about Sam, and he handled their situation incorrectly. I have said again and again that I belive he figured that out in "Swan Song" and that they forgave one another in that episode, without any spoken apologies required (because Winchesters don't need chick flick moments). This was confirmed when Sam's soul came back and they were right there on the same page again, no fuss, no muss. Dean screwed up with Sam, drove him away when he should've helped and he was lucky to get him back. But he's smart, and he learned from his mistakes. So when he found out Cas betrayed him there was the same pain, the same anger. But he wanted to give Cas a chance to redeem himself, and he offered to help because that is what he knows he should have done better with Sam.
I thought this episode was beautifully done overall, but my favourite scene is Dean and Cas' conversation in the living room at night. Partially this is because it's just visually stunning, but more importantly because I think it lets us see this battle between the old Dean of S4 and 5, and this new, slightly wiser version. Dean is pissed and delivering "big brother knows best' ultimatums, but underneath that anger there's a shit load of love. And that was there with Sam too, but Dean got so caught up in his fear and his pride that got in the way. Here, Dean comes awfully close to begging Cas to change his mind, and instead of using the language of logic he uses the language of love and family. It's a really significant shift in his approach to communication in relationships.
And this change isn't an insult to Sam - far from it. It's a compliment. Because this change has happened because of Sam.
It's no secret that I find all the Sam vs. Dean arguments frustrating. I feel the same way about related Sam vs. Cas arguments. I don't think we should be keeping score of who owes whom an apology, especially after S5. I think the larger emotional arc of the story seems to be about Sam and Dean letting go of that tendency in themselves. To stop figuring out who was John's favourite son, who's a better hunter, who started what wars or whose fault any single tragedy is. Because none of these questions have a simple answer, and trying to solve them hurts far more than it helps. Now, Sam and Dean finally seem to see themselves as a real partnership (and it's working out well), and I couldn't be more thrilled about that. That Dean is trying to extend that to other people he cares about only makes sense.
I'm just really, really hoping Cas catches on in time.