I think it's the contrast that gets you. You have this incredibly rigid, brutal world of codes and violence, and then you drop this fragile, intimate connection right in the middle of it. The emotional stakes aren't just about 'will they love me,' they're about 'will loving me get them killed' or 'will my duty force me to destroy the one good thing I have.' The tension isn't just romantic misunderstanding, it's life and death. It makes every quiet moment feel stolen and precious.
Shows like 'Gangsta.' and even parts of 'Banana Fish' play with this, though that's more mafia. The appeal is in the character's internal war—their loyalty to the clan versus their loyalty to their heart. Watching a hardened character, someone who's seen and done terrible things, struggle to be gentle, to be vulnerable, is just... it hits different. The romance feels earned, not just convenient.
Honestly, sometimes the actual romance is almost secondary to watching that armor crack. You're waiting for the moment they choose the person over the oath, knowing the cost will be catastrophic.