Is Dean’s siren Sam-coded more than man-coded?
The siren is trust and esteem coded more than either of those when you get right down to it.
4.14 opens with Dean waking up and hearing Sam talking to someone on the phone about apocalyptic signs. Sam and Dean are working the apocalypse together. There's no reason Sam shouldn't share who he's talking to with Dean. But when Dean asks him why he was up so early, Sam tells him he was on the can instead of "I was talking on the phone to x about y". Sam's hiding things—apocalyptic things, which 1) they're supposed to be working on together and 2) greatly impact Dean's life, given he's been told by some very scary and intimidating people way above his weight class (not that that's ever stopped him from fighting) that he's supposed to be one of the star players in the apocalypse. Dean has had angels breathing down his goddamn neck about the apocalypse for months, but they won't really tell him what's going on and what he's supposed to do, and that's terrifying. It's objectively scary to have foreboding prophecies written about you and angels who you really don't trust hanging around and ordering you around while keeping you on a strict info diet. Dean is worried, and information is one of the things that would really help him figure out what to do, and instead of being in Dean's corner helping him figure out what's going on, Sam is behaving as another person keeping Dean on a info diet—repeatedly. He leaves Dean in the dark, while Dean copes with nightmares and an apocalyptic destiny alone, certain that there are ways in which he is being lied to and used, but unsure what those ways are. Sam is holding extra puzzle pieces and won't share them, and that hurts. Sam is supposed to be the first person Dean can count on for support, and he won't even do Dean the courtesy of sharing what he knows.
Add that there's low reciprocity. Dean opened up to Sam about this big huge trauma—what happened in Hell, and Sam won't even tell him who he's talking to on the phone. Dean is starting to connect his time in hell with Sam's secrecy and sidelining, and he's going to find out at the end of this episode that he's right to—that Sam sees Dean's trauma as a sign of weakness and a justification for excluding him.
So Sam sidelining Dean on apocalypse business is the opening of this episode. Over the course of 4.14, Sam also sidelines Dean on the case of the week, ordering Dean to babysit Agent Munroe. This isn't the first time something like that has happened this season. For example, in 4.07, Sam orders Dean to stop a hoard of monsters surrounding a group of civilians alone then promptly leaves him with that bag, because he wants to face Samhain by himself. Dean is picking up on all of this.
Later in 4.14, Dean finds out the person Sam was talking to on the phone was Ruby, and that introduces more hurt. Dean has never trusted Ruby, and he has perfectly legitimate reasons. That said, Dean actually stowed his shit anyway. In 4.09, he plainly and maturely communicated with Sam that he wanted to understand how him and Ruby started working together and why he trusted her after everything. He also accepted what Sam said, and proceeded to demonstrate that he was willing to work with Ruby. They worked together in 4.10 and Dean even saved her life (for a second time). He even thanked her. So Dean is putting in the effort to ignore his instincts and give Sam trust, but Sam ignores those olive branches and still keeps Dean in the dark after.
This lack of reciprocated trust from Sam becomes connected with esteem. For one, because Sam is so bad at lying and obscuring his secrets that it's actually insulting that he thinks Dean's that stupid and unobservant. For another, Sam's secrets communicate that Dean isn't considered of value in the mission. At the end of this episode, Sam will say that—that it isn't about being worried Dean is a loose canon. It's about Dean being too weak to be included.
OK, fine. You know why I didn't tell you about Ruby, and how we're hunting down Lilith? Because you're too weak to go after her, Dean. You're holding me back. I'm a better hunter than you are. Stronger, smarter. I can take out demons you're too scared to go near. [...] You're too busy sitting around feeling sorry for yourself. Whining about all the souls you tortured in hell. Boo hoo.
Add that all this is crap projection. Add also how cruelly this line tramples on the high degree of trust Dean gave Sam by sharing his trauma—something Sam begged him to do! Now Sam is using it as a weapon to justify keeping him in the dark and sidelining and lying to him—and he doesn't stop doing it!
How does Nick Munroe (the siren) gain Dean's trust enough to get him to share a flask?
He spent quality time with him, talking about basic, normal, non-work things Dean enjoys (music, cars) with interest instead of derision.
He didn't treat Dean like an ignorant idiot (Sam is surprised that Dean knows what The Odyssey is in this episode).
He treated Dean with professional courtesy, letting him in on the case detail of the flowers he'd discovered.
When Dean had a hunch, Munroe told him he thought his hunch was crazy, but he trusted him enough to agree to see where his hunch went anyway (and that simple and very no-big-deal moment gets Dean sharing his flask).
Like. Look how simple and basic this show of "trust" is:
MUNROE: So you think... what? She's drugging these guys?
DEAN: Pretty much.
MUNROE: Uh-huh.
DEAN: I know how it sounds.
MUNROE: You sure about that? 'Cause it sounds like crazy on toast. All these different strippers, they're magically the same girl? But then they're not strippers at all, it's Dr Quinn.
DEAN: It's kinda hard to explain, but I have my reasons and they're good ones, so you're just gunna have to trust me on 'em.
MUNROE: Yeah. OK. I guess.
DEAN: (Surprised) Thank you. That's actually nice to hear.
Dean isn't asking for the moon here. He's absolutely fine with skepticism—with someone not agreeing with him. He's just looking for his hunches to be worth a short stakeout (and while it's not the case here, Dean's hunches are often right and often not believed—to the point I have a whole tag!)
Note that the spell happens through the saliva. Dean and Munroe hanging out up until that point is not Dean being bewitched, or trying to get back at Sam, or wishing Muroe was his brother. It is literally just Dean being treated with basic human and professional courtesy by someone for a few hours during a period in his life where Dean feels undervalued, lied to, and used by most people in his life. The siren knew his usual M.O. (always presenting himself as a woman, always an exotic dancer) wasn't going to work. Dean was a hunter and wasn't going to kiss some random woman in the strip club knowing there was a siren around, so instead, he changed his approach, presented himself as a male to avoid suspicion, then befriended Dean, hanging out with and working with him over the span of a few hours—just enough until sharing a drink didn't set off any alarm bells. It is only after Dean is exposed to his saliva that the siren casts himself as a replacement brother gloating that he's sooo much better than Sam, because getting someone to kill a loved one over the big and small things that bother you about them (seeing HIM as better) is the siren's whole M.O., and Dean doesn't have an S.O., but he has a brother who has kind of been a dick lately (who is also another hunter who the siren needs dead, thus infecting him as well and trying to get them to kill each other).
Obviously, people can certainly play in the sandbox further. But like... What Nick does to get Dean to trust him enough to share the flask is literally so basic, and that's sad in its own way.