You know, actually, there's one distinctive trait, a very simple reason that had me fall in love with sskk.

They meet each other for the first time; they don't hate each other. Akutagawa was just doing his job (rip), and Atsushi's main sentiment towards Akutagawa was fear- hatred too, maybe, but nothing more than any other mafia operative. They don't know each other yet.

Then, they get to know each other. They start to understand each other's reasons and motivations as they share-scream them with each other while fighting. They start to understand each other, and that's when they start hating each other like they've never hated anyone before. But why is that? Where does all this loathing come from? It's quite simple: because as soon as they started knowing each other, they started seeing themselves in the other; and the person both of them hate more than anyone else is themselves. You don't even need to check how both of them have themselves listed as their own major dislike on their respective profiles to perceive that: it's already more than evident by simply taking a look at their self-destructive tendencies, need for validation, null self-esteem they've been carrying through the whole manga. They see themselves in the other, end they hate themselves, so they hate the other too.

But then. Then they start to see something else too. They start to see how broken the other is. They start to see how kind the other is. They start admiring how strong the other is!! And they start questioning, how is that person not worth of love? Isn't it beautiful how they lived through hell, yet they're still standing? And finally!! A slow realization makes its way through them. If they are worthy of love, and they are like me- doesn't that mean I'm worth of love too? If they're worthy of living, does that mean that so am I?

And that's how their feelings of hating themselves, hating each other, loving each other, loving themselves come full circle. That's the thing I love about this ship: how the journey towards learning to love each other is actually a journey to learning to love themselves. And I think that's beautiful :')