Since you're mostly positive I feel like writing this to you maybe you have a different pov. But I'm really disappointed in Dean and Dean/Castiel. I feel like Dean barely cared about Cas being possessed, the end scene had him be way more distraught over the people that were on the ship than Cas. And next week he goes to "pay his respects" for a stranger and needs a "break" and is in totally good spirit watching a wrestling game even though he knows Cas is possessed.
I actually have to say that I am quite surprised anyone could think after this ending that Dean does not care about Cas, but just to explain more in depth, let me untangle this a bit and bear in mind that I only watched the episode once this morning and didn’t have time to re-watch yet. So this all is what I remember and my memory may be a bit spotty.
That being said, there is one thing that imo absolutely needs to be taken into consideration to put things into perspective. This entire episode focused deeply on Dean’s growing depression and feelings of inadequacy and feeling like being unable to make a difference. Dean’s never been in a situation as desperate and hopeless as he is now with his free will and agency severly compromised. Dean has always been the one to shoulder Sam’s burdens, be strong. And even if Dean has always been a lot more fragile than he let on, the thing was that he could make himself believe he was strong. Of course it was playing a role more often than not but in a twisted way and as unhealthy as a method as it was, it helped Dean to focus. This is completely gone now. And Dean is seriously struggling with allowing himself to be weak and be “carried” by Sam. His line about feeling bad that he would do that to Sam at the end of last week’s episode spoke volumes in that regard. And it is so important for Dean to learn that he has value and is important and loved unrelated to his usefulness by which he often defines himself - and Cas too for that matter. It is vital for Dean to realize there is nothing wrong with being “weak” or to be in need of help or support.
Dean’s entire life has been a series of suppression and distraction, a series of different ways to numb the pain and trying to forget about his self doubts.
When the brothers and Lucifer were talking about the time travel (him volunteering could also be read as a sign of him being way to okay to die, because he thinks he is slowing the others down) Dean said that he needed to do this. And as the episode revealed he needed it to prove to himself that he can still kick ass, be a good hunter, matter. It was a way of Dean trying to buld up some self esteem again, unfortunately the opposite turned out being the case, any little bit of hope and self esteem has been crushed. We have seen Dean hopeless and sad and tired a few times before, but I don’t think we have EVER seen Dean be this utterly broken as in this moment on the pier that is a perfect visual for how it looks within Dean: close to crumbling. This mission in which he wanted to prove to himself more than to anybody else - because in the end Dean is his own worst enemy, his worst critic and of course the one who knows best how often he fakes a smile or makes a joke to gloss over the pain and the real issue - he happens to be as he himself said “just a witness”. And that’s exactly how he feels right now and man, Jensen did one hell of a job to portray it.
He made a suggestion and wanted to help Delphine and the ship to be saved, but she was the one to put her foot down. In a very twisted way this may have confirmed to Dean just once more that he is not just weak and vulnerable when it comes to Amara, but in general so that people think they need to take care of him. Which is good and wonderful that people do that, but it’s exactly that Dean has such big issues with.
Anyway. So Dean has all this happening fresh in mind (and the bystander argument does loomingly feel like when Amara may use Dean against his will, he may be unable to resist, but may well inside be distraught over it, because like Abaddon once threatened Dean, she would make Dean watch what she will make him do) as well as all of the trauma and stress that has been repeatedly raining down on him for a better part of 3 seasons, so he is truly at the end of his rope here.
Dean has trust issues left, right and center. And the only people he unconditionally trusts are his brother and Cas. The entire preparation talk showcased how very much he trusts, values and confides in Cas. There is not a doubt in his mind that Cas would not stand by his side, not one. And it’s because of that, that the revelation of Cas being possessed by Lucifer really does the number on Dean that we see in the ending scene.
This is Dean Winchester we are talking about, of course he fees responsible and guilty that he could let that happen to Cas, on the other hand he’s conflicted because he opened up to Lucifer of all people, laid out everything and he was used and played once more.
So all this I am sure is raging behind Dean’s eyelids when he sits at that destroyed pier, everything he has been through weighing on him and then there’s the cherry on top with Cas. That was what broke the camel’s back, what made the levee break. The one constant aside from his brother gone, the only person, who he could confide in in a way he could not or did not want to with Sam seems lost, this is the moment of Dean truly feeling utterly alone, paralyzed. It’s exactly how in 11x07 “Plush” the mother described how she felt when her brother was accused of doing inappropriate things to kids.
Dean sits there mourning so much more than just Cas, but also everything that connects with that, safety, understanding, an ally. And I’m sure thousands have noted this before, but of course the pier has a special connection for Dean and Cas. And while this is all subtext and a place Dean may have chosen subconsciously, it doesn’t change anything about the fact that these tears that he is fighting against there are partially also tears he sheds for Cas. He fight against the tears, because he does not live up to any of the parts of the Winchesters motto anymore:
He can’t “hunt things”, because Amara makes him numb and a shell of himself unable to fight her off and that has been strengthened by so many MotW episode this season that saw Dean unable to help himself. And neither can he “save people” (btw this is not my opinion this is how I think Dean thinks). Quite the opposite. An entire submarine died for him, the Hand of God was used to save him and his best friend is lost to Lucifer. And Dean knows that he is not strong enough to truly be able to help Cas get back, because he is so vulnerable at the moment. Let that sink in for a moment, think about the dread and self loathing Dean feels over this. Dean crying at that pier, barely saying a word is not a sign of him not caring, it is a sign of him caring so much that it paralyzes him, tortures him to the point he seems almost blank.
So no, I don’t think Dean just for a second did not care enough about Castiel or him being possessed (btw I am not sure I remember right, but Sam did not tell Dean yet that Cas said yes willingly, Dean thinks he was tricked, if he finds out that Cas said yes possibly for the same reasons and the same feelings he harbours at the moment, be sure there to be more tears and heartache), I think he cared too much to be able to even filter it anymore - and just once again HATS OFF JENSEN, AMAZINGLY DONE.
And Dean “enjoying” the wrestling in the next episode? Tbh, I don’t think this is a sign for Dean not caring, this is a Dean who tries to suppress, to distract, because if he wouldn’t, I am pretty certain Dean would jump off the next cliff within a heartbeat. This is “an act” just as much as him picking up someone on Valentine’s day was “an act” and technic to relieve the pain. Look back at 11 seasons of Supernatural and tell me how Dean deals when he is scared out of his mind, grieving and depressed (aside from drinking more heavily than usual), yes his usual m.o. is to throw himself into something to not think about how much it hurts. Him doing that doesn’t show he doesn’t care, it shows that Dean doesn’t know how much more blows he can take.