(None of that The Last logic please, kishi didn't write that movie with his own hands. :\)
Hashirama's MISTAKE^ , which Naruto resolved to never repeat.
Instead of getting revenge (causing more fighting,) he forgives and brings all sympathetic villians to the good side. (Itachi, Obito, Madara, Nagato/Konan, Neji, etc)
But suddenly when his greeeat grandma shows up:
He seals this one permanently without trying to force her side of the story out of her and then convert her. This last one.
he did convert Sasuke but, he outright got rid of Kaguya without redeeming her. Just like Hashirama simply gave up on converting Madara and simply "killed" him.
This could potentially be the start of Naruto's CHANGE into a "the end justifies the means" type of person, and that's how we gradually ended up with a semi-heartless Hokage who'd rather endure, forcing himself to take things SERIOUSLY and do paperwork all day instead of goofing off and playing around with his son+daughter+wife and doesn't give a kitten about how Sakura and her family is faring because he needs to endure his Hokage duties. (Like a parallel to Hashirama's mistake when he "killed" Madara.)
If that were the case, it makes perfect sense why NaruSaku didn't happen and... Sakura would rather choose Sasuke over a guy like that, she rooted so long 600 chapters for and then he just GAVE UP. She would not want to be together with that Naruto. And I guess it's a wise decision, given this isn't the same Naruto?
What do you think?
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"NS not happening" isn't the reason Naruto changed, I think this is the reason^
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I also wonder, WHY, would he do this to Kaguya?
He didn't even try to talk to her to hear her motivations from her own mouth!
So was it because Kaguya was too difficult an opponent and too incomprehensible that he just gave up because he didn't want to risk the lives of his friends over someone THIS unfathomable? Like Hashi didnt want to risk Konoha.
Or it could have something to do with Neji's death. Hinata told Naruto to
not go back on his words (a.k.a. endure it) and that Neji's memory lives in their hearts.
Obito (and inadvertibly Hinata) also rubbed it in Naruto's face that he could do NOTHING when Neji died in front of his eyes. (This is the difference between how Jiraiya died)
Thus saying that Naruto is useless and there are some things he can't stop even if he is physically there: therefore, Naruto must endure it and move on. He must do it FOR his comrades because his own life isn't his. (like Hashirama must do it FOR the village)
I can imagine either Sakura or Sasuke saying "You changed, Naruto" or just thinking it inside either their mind or subconscious. And this mutual feeling in regards to Naruto made SS possible, plus they were together for the Kaguya battle so they knew what happened.
"You changed, Hashirama," -Madara | = | "You changed, Naruto," -Sakura (+Sasuke?)