I've always thought his quotes show this gradual shift from external duty to internal conviction. In the early arcs, lines like 'No matter how many times I'm knocked down, I will stand back up!' feel like pure stubbornness, a kid gritting his teeth for his sister's sake. It's reactive, born of desperation. Later, after facing so much suffering, both human and demon, you get 'I won't let anyone suffer, not even a demon'—that's a massive leap. He's not just fighting for Nezuko anymore; he's internalized a philosophy, one that's almost naive in its compassion but becomes his core strength. That complexity is what makes his journey stick with me, the way his kindness hardens into something unbreakable, a moral compass that guides the blade.
Some people find his speeches repetitive, but I think missing the subtle changes means missing the point. Early Tanjiro shouts about protecting his family. Mid-series Tanjiro talks about understanding pain, like with Rui or the Hand Demon. Endgame Tanjiro's words are quieter, steadier, grounded in that accumulated understanding. It's the difference between a battle cry and a vow. You can chart his entire emotional arc just through those key lines, watching the boy become the pillar.