In the fanfiction, it could be related to a magical or cursed situation. For example, he might be under a spell that turns him into a girl. And while in this form, he decides to make the best of it. He may start to appreciate the different ways of thinking and acting that come with being a girl and ultimately decides to continue in this form for a while, learning and growing in the process.
Perhaps in the fanfiction, Naruto wants to experience the world from a different perspective. By becoming a girl, he can understand the challenges and advantages that girls in the Naruto universe face. It could also be a way to gain new allies or friends. Some female characters might be more inclined to trust and befriend a girl, so Naruto sees this as an opportunity to expand his social circle and gain more support for his goals.
One possible reason could be for a mission. Maybe in the fanfiction, there is a mission that requires Naruto to go undercover as a girl. This would allow him to access certain places or information that would be otherwise unavailable to him as a boy. Another reason might be for self - discovery. Naruto could be curious about what it's like to be a girl and decides to explore that side of himself in the fictional world.
One possible reason could be a traumatic event. Maybe he witnessed something so horrible that it made him shut down his emotions to protect himself. For example, if he saw all his friends killed in front of him, it could be too much for him to bear and he just numbs himself.
One possible reason could be that Adrien is being blackmailed by Hawkmoth. Maybe Hawkmoth has some information or has threatened someone Adrien cares about deeply, like his friends or family. So, in order to protect them, he decides to help Hawkmoth, even though he knows it's wrong.
Perhaps a dark power corrupts him. In the fanfiction, there could be some sort of dark chakra or evil entity that latches onto Naruto. This entity could slowly eat away at his emotions, starting with his heart, making him gradually lose his ability to feel empathy and turn heartless.
Magic could be one reason. Just like in fairy tales, a wrong spell by a wizard or witch can force the transformation. Another reason could be some sort of scientific experiment gone wrong. Scientists might be trying to achieve something else, but it results in this unexpected change.
One possible reason could be cultural or traditional beliefs in some extreme cases. For example, in a very patriarchal society where having a girl is highly desired for certain family inheritances or religious practices, a family might try to force a boy to live as a girl. Another reason could be related to some psychological issues of the people forcing it. Maybe they have a distorted view of gender and think they can 'create' the gender they want.
Perhaps he makes a deal with a powerful entity. In order to gain some extraordinary power to save the village or his friends, he has to sacrifice his voice. It could also be due to a side - effect of some new jutsu he tries to master. The jutsu could be so powerful that it takes away his ability to speak as a trade - off for the new abilities it gives him.
She might have realized how much Naruto has sacrificed for her and their friends. His selfless acts could have made her feel that she needs to protect him in return.
Perhaps Naruto has a big dream, like becoming Hokage. She might think that in a world where male leadership is more common, hiding her gender would increase her chances of being taken seriously. People might be more likely to believe in a male's ability to lead rather than a girl's, so she hides it to pursue her goal more effectively.
Perhaps the discovery of some dark secrets about the ninja world could make him ruthless. For example, if he finds out that the village leaders have been using him or his friends for some immoral purposes all along. This new knowledge could shatter his world view and make him question all his previous beliefs, leading him to become a more ruthless individual who is out to change or even destroy the existing order.
Well, perhaps she has been exposed to a lot of media that portrays bimbos in a glamorous light. She might think that by becoming a bimbo, she can gain more attention and have a more exciting social life. It could also be that she feels restricted by her smart - girl image and believes that being a bimbo will liberate her from the expectations that come with being intelligent. For example, she may not want to always be the one with all the answers and instead just have fun without the pressure of being smart all the time.