Another reason might be related to a plot device in the fanfiction. The author could be using the loss of the mask to create a turning point in the story. It might lead to new relationships being formed or old ones being tested. For instance, his friends might react in different ways when they see his face without the mask, and this could drive the story in different directions.
One possible reason could be the loss of his friends. Naruto deeply values his relationships, and if something were to happen to his friends like Sasuke leaving again or getting seriously injured, it would shatter his happiness. Another reason might be if he fails to protect the village. Naruto has always seen himself as the protector of Konoha, so if he fails in this duty, his sense of purpose and happiness would be lost.
If Naruto loses his mask in this fanfiction, it might change the power dynamics. His identity being more exposed could make him a target. The fem Kyuubi might have to step up and use her powers to protect him. This could lead to a closer bond between them, and they might start to rely on each other more in the face of new threats.
The fem Kyuubi might feel a sense of protectiveness. Seeing Naruto without his mask could make her realize how vulnerable he is. She may then use her powers to safeguard him from any potential threats that could take advantage of his exposed state.
He might become more confident. Without the mask, he no longer has something to hide behind, and this new - found openness could boost his self - assurance.
One reason could be betrayal by his friends. For example, if someone he considered a close ally made a deal with his enemies behind his back, it would shatter his trust. Another might be lies from the village leaders he respected. If they hid important information or manipulated him for their own gains, Naruto would start to question who he could trust.
One possible reason could be for disguise. Maybe he is on a secret mission and doesn't want to be recognized easily. In the Naruto world, there are many dangerous foes, and wearing a face mask can help him blend in or hide his identity from those who might be targeting him.
If someone were to insult or harm Hinata, who he loves deeply, Naruto could lose his temper. He's very protective of those close to him, and any threat to Hinata would likely trigger his anger. He has a strong sense of justice and loyalty, so seeing someone he cares about being mistreated would be a big deal.
His friends would be affected. For example, Sasuke might see Naruto in a new light and realize that they are more similar than he thought, both having their own inner struggles. Sakura would also be more empathetic towards Naruto and might try to be a better friend, offering more emotional support.
Well, it could lead to a moment of great vulnerability. His friends might see a side of him they've never seen before, perhaps a more troubled or uncertain Naruto. This could deepen their relationships as they offer him more support.
Maybe he was protecting someone in Wave. He's always the self - sacrificing type, so he could have put himself in harm's way to save a villager or a friend.