Deep down, i believe the two brothers are both protagonists, and that's a given fact, but i have another more comprehensive reason Sam is the protagonist.
I dont wanna delve into it, but it's something along the lines that Dean has always existed as an ultimate extension of Sam. His standing within the narrative is that of a soldier, not a leader; he exists to protect (sam) and constantly needs an external drive (which sam fills) to do anything.
Dean is also heavily biased and prejudiced. He is not really a protagonist for a story who'd lead/take all under his wing bc he wouldn't. He is definitely a protagonist in Sam's life but not in the supernatural story.
He doesn't hold the axis of the general narrative as Sam does, and as he does hold Sam's.
Yea they both dont make the best selfless protags but Dean is by far the lesser chivalrous out of them, he is also unwilling to take that burden canonly, he HATES taking care of other people his role as a soldier/guardian has always kinda just extended to Sam and ended there that's why he says there's no me if there ain't no you to Sam. Dean IS the ultimate extension of Sam. Without Sam, he ends.
like it is--one of thee wildest parts to me is that sam wasn't... I mean no okay he was suicidal. but he wasn't like wed to the concept of his own immediate death. he was probably clinically depressed and he was extremely willing and ready to die in order to prevent future harm to others, but his expressed wish wasn't just To Die as an end in itself and he literally out loud twice agrees that no okay he'll live then. (I think this is muddied later by the way dean and sam both talk about these moments like dean forced sam to live, but genuinely onscreen he didn't. he asked and sam said okay. dean's requests have an outsize impact on sam but dean didn't actually prevent sam, in the moments he was just verbally asking, from dying. right up until he does force sam into something.)
so i mean. what happens onscreen isn't 'dean prevents sam from committing suicide and that's a problem.' it's 'dean prevents sam from dying in the line of duty, which is fine, and then when sam almost dies anyway dean reviews sam's dnr form which says "okay you can bring me back except not if you have to do it with this one drug. i've tried that family of drugs before and it causes me physical and mental anguish so severe it's not a circumstance I, a rational adult person, am willing to live in. so in THAT case I'd rather not be rescuscitated." and dean, who very specifically can talk to sam because that is how he accomplishes this entire thing, tells the man in a lab coat he found in an alley "ok i'm going to distract him and you pump his veins full of this stuff. we're not going to ask him whether he feels differently now or is willing to try it this one time. he just told me he wants to live so maybe he would budge but we're not consulting him on this. I WILL be holding him down." and then they both kept doing that for months despite the fact that they could at any point have asked sam hey, you seem loads better, do you think you'd be down to try this lifesaving drug now or is that still a hard no?" and that is. in fact. a problem.'
season nine pinnacle supernatural experience because dean's big crime of the season is being party to sam's thin-metaphor-for-rape & saying the whole time damn sam is going to be upset when he finds out about this. and sam's big crime of the seaon is being upset when he finds out about this.
you know that line in season 7 when sam is talking to marin about seeing/hallucinating her brother’s ghost and how at first it must’ve been nice because of how much she must have missed him and sam’s like also currently in the psych hospital for hallucinating lucifer. you must have missed him. at first it was nice. how do you spend an eternity in the cage with someone and not miss them a little bit? how do you even classify all your feelings about that person when they were all you had and so evil and hurtful and reprehensible but also the only source of relief and respite and conversation and connection?
so sam and gadreel are talking about lucifer and sam says well you must hate him and gadreel says yes but i love him too. we all did. sam says sure yeah BEFORE— and gadreel says yes, and during, and after. he was wonderful and we loved him and he is terrible and we love him. i could never forgive him and i miss him every day. and sam has to excuse himself to go have a panic attack lying facedown on the ground under his bed