Honestly, I always saw Naruto's smirk as this armor he'd put on, a reflex he developed way back when everyone in the village looked right through him. It's not a real smile, not at first. It's this defiant, messy grin he flashes right when things are at their absolute worst—when he's beaten down, out of chakra, and the villain is monologuing about how hopeless it all is. That's the moment his smirk shows up. To me, that's the core of it: it's a rejection of the given narrative. The world says he should lie down, so he smirks and gets back up. It's performative, almost theatrical, like he's starring in his own underdog movie even when nobody else is watching.
Later on, it evolves, but the foundation is still that stubborn refusal to accept defeat. When Pain has the village destroyed and asks him about pain, Naruto doesn't give a heroic speech right away. He grits his teeth, and there's that smirk—raw and angry—before he finds his answer. It's like his fight instinct manifesting on his face. It's also a communication tool, a way to reassure his teammates without words. He'd smirk at Sakura or Shikamaru not because he has a plan, but because he needs them to believe he might. It's leadership through sheer, dumb, contagious bravado. The smirk says 'I haven't figured it out yet, but I will, so just watch me.' It's less about confidence and more about a desperate, unwavering commitment to prove everyone wrong, including himself.