DEAN: No, I need you to be safe, Sam, okay? That's what I need.
SAM: What? What am I – when are WE ever safe?
DEAN: This is different.
SAM: How?
DEAN: Because of the three trials crap – God's little obstacle course. We've been down roads like this before, man – with Yellow-Eyes, Lucifer, Dick friggin' Roman. We both know where this ends – one of us dies... Or worse.
SAM: So, what – you just up and decided it's gonna be you?
DEAN: I'm a grunt, Sam. You're not. You've always been the brains of this operation.
SAM: Dean—
DEAN: And you told me yourself that you see a way out. You see a light at the end of this ugly-ass tunnel. I don't. But I tell you what I do know – it's that I'm gonna die with a gun in my hand. 'Cause that's what I have waiting for me – that's all I have waiting for me. I want you to get out. I want you to have a life – become a man of Letters, whatever. You, with a wife and kids and – and – and grandkids, living till you're fat and bald and chugging Viagra – that is my perfect ending, and it's the only one that I'm gonna get. So I'm gonna do these trials. I'm gonna do them alone – end of story. You're staying here. I'm going out there. If landshark comes knocking, you call me. If you try to follow me, I'm gonna put a bullet in your damn leg.
Okay so there are things about this speech I want to tie back to things that happened during the season.
First, Sam's comments back in 8x03 "Heartache" characterizing Dean as someone who would be happiest hunting alone for the rest of his life, and the implication that Dean's life doesn't have meaning.
DEAN: I know where I'm at my best, and that is right here, driving down crazy street next to you.
SAM: Makes sense.
DEAN: Yes, it does.
SAM: Or... maybe you don't need me. I mean, maybe you're at your best hacking and slicing your way through all the world's crap alone, not having to explain yourself to anybody.
DEAN: Yeah, that makes sense, seeing as I have so many other brothers I can talk to about this stuff.
SAM: Look, I'm not saying I'm bailing on you. I'm just saying make room for the possibility that we want different things. I mean, I want my time to count for something.
Dean internalizing portions of this conversation pairs well with Dean's comments about having nothing waiting for him, and his life meaning less than Sam's.
Second, in 8x12 "As Time Goes By", the many classist jabs thrown at Dean and hunters in general by Henry, implying that Sam and Dean are disappointments to him/to the family legacy, that hunters are animals, that they are unintelligent.
HENRY: You're not. Are you? Hunters? Well, hunters are... Hunters are apes. You're supposed to – you're legacies.
DEAN: Why, because we're hunters? What do you have against us?
HENRY: Aside from the unthinking, unwashed, shoot-first-and-don't-bother-to-ask-questions-later part, not much, really.
SAM: You know what? Wait a second. We're also John's children.
HENRY: You're more than that, actually. My father and his father before him were both Men of Letters, as John and you two should have been. We're preceptors, beholders, chroniclers of all that which man does not understand. We share our findings with a few trusted hunters – the very elite. They do the rest.
(Note also that Henry positions himself as above the activities of hunters also by refusing to help dig the grave).
Henry makes a jab at Dean specifically before he knows who Sam and Dean are:
HENRY: I'm quite certain this is all beyond your understanding, my alpha-male-monkey friend. And violence will not help you comprehend this any easier.
Dean makes it clear he sees all of Henry's comments as claims of disappointment in them and commentary on their worth.
DEAN: Listen, I understand that this is not your idea of a happy ending, okay, and that – that you're disappointed that me and Sam are mouth-breathing hunters. But you know what? We stopped the Apocalypse.
Dean initially rejects the Men of Letters legacy (no doubt in large part because of Henry's distasteful rhetoric). He appreciates the bunker itself in 8x13, but says the Men of Letters probably "made crap up". He cautions Sam against thinking that the Men of Letters were as superior as they believed themselves to be. In that episode though, they meet Aaron Bass, and Aaron finds meaning in the family legacy of the Judah Initiative, feeling closer to his grandfather and feeling a sense of duty and pride in preserving his family's legacy and continuing their important work. After that, when they come back to the bunker, Dean again takes note of Sam's high interest in the Men of Letters archives, but this time with pride and approval.
DEAN: What're you doing?
SAM: Ordering – I'm making a, uh, card entry for our, uh, copies from the Thule's red ledger for our collection.
DEAN: So, uh, what? Aaron's a J.I., and... you're a Man of Letters now? Is that it? [...] Good.
Dean swaps out the beers he had just grabbed out of the fridge for whiskey, and hands Sam a glass as if they are celebrating something Sam has accomplished. Dean notably does not make himself a part of that Men of Letters legacy. He smiles at Sam with pride, but sits facing away from the work Sam is doing, with his feet propped on the table, separating himself. In the beginning of the following episode, Dean jokes that he isn't going to be the one to read and catalog all the books in their archive so it's good that Sam is doing it.
So throughout these episodes, we see Dean being confronted with a couple of negative messages about himself:
His life does not count for anything/exists at a lower level of importance.
Dean internalizing these messages to some degree explains "I'm a grunt" and positioning Sam as "the brains of the operation" when a very well-documented history shows that (as Sam himself will say later in the episode) Dean is a genius (see my smart dean tag). Of course—Dean's had plenty of opportunities to internalize messages about his intelligence, as it's consistently been used as a way to insult him since the very beginning of the series.
Third, Dean becomes more focused on nurturing and peace with the introduction of the bunker (or really, one could argue starting with 8x11 "LARP and the Real Girl", during which he wants more than anything to spend some time enjoying life, and worries that Sam is unhappy and working too hard). The episode after they find the bunker (8x13) opens with Dean coming back from a solo drive to check on Kevin. He mopes about having to leave to go on a hunt right after getting back home. In 8x14, Dean is excited about having his own room, decorating that room, and having access to a real kitchen. He makes homemade burgers for himself and Sam, and then when they go to check on Kevin, Dean goes to get him food after learning Kevin's been subsisting on hot dogs. In 8x13 "Everybody Hates Hitler", Dean seems to actually consider Aaron's fake proposition. All of this feels inconsistent with the idea of Dean as truly suicidal. As Sam points out in his returning speech in 8x14, Dean has things and people that make him happy.
SAM: You have friends up here, family. I mean, hell, you even got your own room now. You were right, okay? I see light at the end of this tunnel. And I'm sorry you don't – I am. But it's there. And if you come with me, I can take you to it.
So then where is Dean's almost aggressive push for his own death coming from? It isn't that Dean wants to die, it's that he's internalized certain messages about his own life being worth less than other people's lives, and those messages are crap. It's that he's been told he is stupider than everyone else, which is also crap. It's also that he feels uncertain about being able to maintain happiness and loves his family over himself:
Fourth, Dean places a photo of himself and Mary together on his bedside table as he decorates his room in 8x14. One implication of the photo is Dean feeling a peace, but another is that the photo represents a past Dean can't ever return to (1x01 "I can never go home"), where he felt loved and safe. Another implication is that the photo connects Dean with the Campbell legacy, while Sam's interest in the Men of Letters archive connects him with the Winchester legacy. In 8x13, Sam positions the Winchesters and the Campbells as "The brains and the brawn", respectively. This is another nod at Dean being positioned as "the grunt" vs. Sam as "the brains of the operation". Yet another implication of the photo is that it connects Dean with Ellie, the girl who sold her soul to Crowley and condemned herself to hell to heal her mother of Parkinson's. Ellie and her mother don't even live together, but knowing her mother is happy and alive is enough for Ellie:
ELLIE: [Crowley] didn't make me do anything. My mom – she had Parkinson's – early stages, and I knew how that story would end. So I took the deal. Ma is retired down in Phoenix now, plays golf every day.
DEAN: That was a stupid move, Ellie.
ELLIE: I did it for my mom, Dean. What would YOU do for your mom?
Dean will do stupid, self-destructive moves to preserve the happiness of members of his family, but like Ellie, he doesn't actually want to die. This episode also brings us back to memories of Dean's own demon deal, and his eventual admittance in 3x10 (after burning bright intending to go out with a bang, pushing himself into sex like Ellie does in 8x14, and saying he is fine with this) that he doesn't want to die—that he wants Sam to save him. Dean cries out for Sam begging Sam to rescue him even after he dies and is actively being tortured in hell. Which brings me to...
DEAN: Because of the three trials crap – God's little obstacle course. We've been down roads like this before, man – with Yellow-Eyes, Lucifer, Dick friggin' Roman. We both know where this ends – one of us dies... Or worse.
Fifth, Dean remembers that the last time "or worse" happened to him, Sam did not look for him, and remembers Sam's own statements about Dean basically being a waste of space from 8x03, where Sam was numb to everything (and Dean didn't/doesn't know Sam was lying to himself and Dean and he won't actually know Sam cares until the end of season 9 lmfao).
Finally, Dean possibly internalizes a little blame for Sam leaving Amelia, even though it has nothing to do with him and he wasn't the one who issued ultimatums. Sam chose hunting while Dean told Sam to go back to Amelia, but Dean worries Sam chose wrong in the aftermath (8x12) and sees the hunter lifestyle as something Sam has sacrificed for in some way and as something that is possibly stifling his potential (see again 8x03 and Dean pushing him toward the MoL legacy which is also safer work). He also believes Sam blames him (8x06g. Dean potentially sees it as his fault that Sam doesn't have the life he wants, maybe in part because of Sam positioning Dean in 8x03 as helplessly connected to hunting, but this is a fairly consistent hang up for Dean, as seen in 2x20, 6x15, 8x06 (also see this post).
Anyway. Fuck people who claim this speech proves Dean really and truly considered dying young a bloody a happy ending. Apparently "It's the only one I'm gonna get" and "that's all I have waiting for me" weren't enough of a tip off that this isn't actually happy at all. And neither was Sam's horror (also as a repeat from Dean's demon deal and the conversation about dying bloody or sad back in 4x12).