This pairing always felt like someone trying to force two puzzle pieces from different boxes to click, which honestly is where most of its weird, specific tension comes from. Nagito's whole thing is a compulsive, self-flagellating worship of hope, while Sans is defined by a weary, detached understanding of futility after countless resets. Throwing them together creates this bizarre emotional pressure cooker. The core conflict isn't really romantic jealousy or external threats—it's about two utterly incompatible worldviews colliding. Nagito might see Sans's apathy as the ultimate 'despair' to overcome, a supreme challenge for his hope, while Sans would probably just find the whole performance exhausting and vaguely pitiable. The fanfics that hook me explore that push-pull: Nagito's manic devotion constantly meeting a wall of tired sarcasm, forcing him to either escalate or, in rare good stories, actually introspect. Sans, meanwhile, might be reluctantly drawn into caring about someone so actively self-destructive, battling his own learned helplessness. It's less about sparks and more about philosophical abrasion.
Of course, a lot of fics just go for crack or pure smut, which is fun in its own way, but the few that dig into the dynamic's inherent dissonance end up being these strangely melancholic character studies. They're not even about fixing each other, because both characters are fundamentally broken in ways that define them. It's more about two damaged people creating a new, weirdly stable kind of dysfunction together, where Nagito's chaos finds a deadpan audience and Sans's detachment finds something bizarrely, persistently hopeful to observe. That's the heart of it, I think.