Oxytocin isn't just a cuddle/trust/romance hormone. It's also associated with prejudice--with loyalty to your own group and hostility to others. In some experiments, oxytocin exposure raised the amount of punishment that test subjects were willing to impose on out-group members.

So not only does it make you want to bond with someone... it makes you want to hurt everybody else.

Is Aura of Smooth oxytocin? I'd say--no. But, like a lot of drugs, Aura of Smooth probably works on oxytocin receptors, at least in cases where the species involved has them, or an analogue of them. It's an artificial substance, much stronger, that was never meant to be in the brains of the poor canons to begin with. It's not like the gentle feelings of attachment that oxytocin brings. Aura of Smooth creates more of an addiction and an inability to think for oneself. It is literary parasitism.

But Sues do even more than that. They can influence even characters who are incapable of attachment. They can romance Voldemort, force normally sociopathic characters to care about them, and even entrance robots, aliens, and inanimate objects. Their influence isn't a matter of simple chemistry--they warp the story directly. A Mary Sue isn't just a simple parasite; she's a reality-warping eldritch abomination.

>Generic "everyone will like you if you use this thing" blurb
>Classy quote by Lao Tzu
>Works though placebo effect
>"Meh" reviews
>Normally 40 bucks
>About 7.4 ml of product

I'll take "Miscellaneous scams" for 500$, please.

That sounds pretty close. But since Aura of Smooth warps the characters' thought processes so all they can think about is the Sue while in her vicinity, I'm thinking there's some Glitter involved in the genuine article. (If so, probably very fine glitter so even the Sue might not know she has a glittery aura.)

At least the assassins spend pretty much all their time around the Aura of Smooth, so I imagine they've built up an immunity by now.