Oh, poor Dean! *sobs profusely* They broke him. Bad angels, bad!
From the first shot in the Impala, Dean is in a very bad place, so much so Sam is shown driving while Dean is withdrawn in the passenger seat. He's crushed over Pamela's death, and that on top of his drama with Sam and the angels have worn him down. He's deeply depressed and doesn't join in on Sam's eagerness to track down a Lilith lead.
It's Dean's outburst after his depressed arrival at just another motel room that sums it up so well. He's upset about Pamela and wants to be left alone. I think the phrase "So maybe you can stop pushing us around like chess pieces for five freaking minutes!" gets the point across. The burdens of saving the world are wearing him thin and there's never a break. The nasty angels take him anyway. Dicks with wings indeed.
They have Alastair, and Dean is the only one that can get answers over who's killing the angels. He must use torture, the stuff he learned while he was in Hell. Dean desperately doesn't want to become that person again. He doesn't want to unleash that animal inside of him that he so loathes. Castiel is sympathetic, but as a good trained soldier, orders are orders. Dean's crushing acceptance that he must put more important needs over his own pretty much kills us dead. He puts duty above all else, a bravery test passed.
His lengthy and agonizing scenes with an unyielding Alastair twist our insides (and his too) about 800 ways. Dean holds firm even when the most crushing news possible happens. He broke the first seal with his first tortured soul in Hell. He swallows the devastating emotional blow but accepts his work isn't done. He goes back to Alastair, who's escaped thanks to Uriel and his "convert angels into Lucifer followers plan." Alastair beats Dean to near death before Castiel saves him.
After coming out of the coma, Castiel confirms that Dean started the apocalypse, and he's the only one that can stop it (I guess knowing the outcome now, that is a true statement). Dean has nothing left. He does the most human thing he's ever done, he cries and admits defeat. Castiel has no idea what to do.
The Monster At The End Of This Book
Thanks to the "intervention" in "It's A Terrible Life" Dean is back, but he's not ready to take on Lilith. Is his confidence shaken or is he being smart?
He calls Sam in a roundabout way reckless, and tries to do the opposite of what Chuck wrote. It doesn't work, especially when Sam is itching for a showdown and refuses to listen to him. What else can Dean do? He does the inconceivable, he prays! Castiel is very pleased by this show of faith and helps Dean in a very sneaky way. Dean has left a big enough impression on Castiel where the angel is now pushing his boundaries.
Dean still hasn't let go of the idea that he must protect Sam. He is willing to do anything, even taking his chances with the wrath of an archangel. He is able to save Sam for now, but soon finds out Sam is more determined than ever to hunt Lilith down. The fight is not over and neither is the danger.
Jump The Shark
With his new found perspective from the last few seasons, Dean without those rose colored glasses gets to see the very worst of his father, and it all comes from his brother. It bothers him a lot to where he's butting heads with Sam much like Sam did with their dad. No doubt when Dean salts and burns Adam's remains he thinks in the back of his mind that he isn't going to make the same mistake like he did with Sam. Trying to bring Adam back would be unnatural. He has indeed learned. Remember this for later.
Dean held incredible composure after seeing first hand what a blood sucking freak Sam has become. He didn't lose his temper or scream and yell, even though that was the final straw for him. That worries Sam a little, especially when he begs Dean to get it out in the open, but Dean doesn't give up the plan. With Bobby's help, it's time to rescue Sam from himself.
Things go really bad with Castiel too. Castiel comes to him for help, but he's dragged off to Heaven before he can talk with Dean. When Castiel returns from being Heaven, he is no longer Dean's ally and friend. Dean is back to square one.
When The Levee Breaks
Maybe Dean's tired of reasoning with Sam, maybe he thinks there's no getting through to him, but locking Sam in the panic room in his mind is the only solution. Dry out like a junkie would and then maybe he'll come to his senses. After all, Sam's erratic behavior, his deception, that's all due to the demon blood right?
I don't think Dean goes into the detox plan knowing all the risks or understanding why Sam is doing this to himself. He seems a little shocked when Bobby says "if he survives." He might not have seen this as more than Sam's misguided need for revenge. His only reason is it's the best way to protect Sam, which is why he makes the deal with Castiel under the air of suspicion.
No, Dean doesn't realize how bad Sam's addiction has grown until Sam has the seizure. Sam has become unrecognizable. Dean could have taken Bobby's advice and given Sam more demon blood, but that wouldn't have gotten his brother back, which is all he wants. Sam is better off dead than becoming what Castiel warned him Sam could become.
That thinking isn't wrong, but Dean lets his anger get the better of him when Sam escapes and gets together with Ruby. When Sam saves Ruby from Dean's attack, Dean is too mad and incredulous at the time to listen to Sam's plea. He thinks Sam doesn't know what he's doing and he even says so. Dean ignores Bobby's advice and hits the hot button, using the "m" word, not realizing that's the one word Sam couldn't hear from him (as we saw in the earlier hallucination). I'm sure he didn't think that Sam would start throwing punches and try to kill him. He probably figured Sam would passively listen to him like he always did, swallowing all that resentment.
All that's left as Sam goes to leave is an ultimatum. If Sam is willing to walk out on family, then Dean can't help him anymore. Sam leaves anyway. Dean takes that to be the ultimate betrayal without understanding why Sam did it. He really doesn't know his brother.
Yes, out of a season of tests, this one Dean failed. (Sam, is no innocent either, so blame is not being assigned in that statement).
Thank heavens Bobby is great at virtual ass kicking. He hands Dean his on platter, telling him that family is supposed to make him miserable and he's a better man than his father, who turned Sam away rather than talk to him. That little pep talk comes just in time too before the angels take Dean away, right when he starts thinking about trying to get hold of Sam.
The end is coming, the final showdown, time to make amends. Except it's too late. Sam won't answer his cell, the angels won't let him leave and they won't give him five minutes with Sam. Zachariah gleefully announces the apocalypse is happening and Sam plays a part. Now Dean is really worried about Sam and what they're going to do to him. The fact that Sam becomes his biggest worry means his priorities are back in line. He hasn't given up on family.
Dean's greatest triumph though is he finally gets through to Castiel. The orders are wrong and not coming from God. Castiel sends Zachariah away and tells Dean the horrible truth. If Sam kills Lilith, the end begins.
I've watched the end of "Lucifer Rising" several times and while Dean is concerned of Sam's well being, he still looks mad just before Sam tells him he's sorry. Not seething, but irritated. I'm dying to see what sort of brotherly exchange comes from that. Me thinks Dean isn't going to forgive Sam so easily. There's too much mistrust and hard feelings. He's going to need to be there for Sam though, since his brother is facing his darkest hour and can't do it alone.
As for Dean's mental state, I think he's more than ready to take on the apocalypse now. He's passed all his character tests. It's time to step up. He's just got to pick up a fallen angel and an addict brother first. Men of destiny gotta do that sort of thing. Bring on season five!