One reason could be for the sake of creating unique power dynamics. If Naruto is an Uchiha, he could potentially have access to the Sharingan or other Uchiha abilities, which would make his battles and character development very different.
Well, it depends on the specific fanfic. There are countless fanfictions out there, and some might indeed have a plot where Naruto has a secret connection to the Uchiha clan. Maybe it's a result of some hidden family ties or a magical spell that makes him related to them in some way.
One reason could be to subvert the expectations set in the original series. In the original, Naruto is seen as an underdog. Making him a secret prodigy in fanfiction turns that on its head. Another reason might be to create a more complex backstory. If he's secretly a prodigy, there could be a whole hidden history about his family or a special training he received as a child that no one knows about.
There are several potential reasons. It could be a result of a psychological experiment gone wrong. Some fanfictions like to play with the idea of Naruto being a test subject, and this could lead to his emotions being numbed. Another aspect could be related to his role as a leader. In some fanfictions, to be a strong and effective leader, he might believe he has to hide his emotions. Additionally, it could be a reaction to the expectations of others. If everyone expects him to be a fearless hero all the time, he might start to suppress his true emotions to meet those expectations.
One possible reason could be a misunderstanding. Maybe Naruto is forced into a situation where he has to take actions that seem like a betrayal to the Uchiha. For example, if there are political forces at play that make Naruto choose between the safety of the village as a whole and the specific interests of the Uchiha clan. Naruto, being a firm believer in the village's unity, might make a choice that the Uchiha perceive as betrayal.
Another reason could be related to his relationships. Maybe in a fanfic, Naruto has a falling out with his friends like Sasuke or Sakura. This emotional hurt can be just as painful as physical harm. The author might use this to show how Naruto grows and overcomes such internal struggles.
One possible plot twist could be that Naruto develops Uchiha - like abilities early on. Since the Uchiha are known for their Sharingan and powerful fire - style jutsu, Naruto might accidentally awaken a form of Sharingan - like power due to being raised in an Uchiha - influenced environment. This could cause a stir in the ninja world as it goes against the normal development of a Jinchuriki.
One possible reason could be that Naruto is so focused on his goal of becoming Hokage. His mind is occupied with training and achieving power, so he seems cold to Sakura. He might not have the time or mental space to show her more warmth.
Maybe he realizes that the shinobi world is too corrupt. There could be political intrigue and power struggles that go against his ideals. He has always fought for peace and unity, but if he sees that the system is beyond repair, he might choose to quit. Also, he could have a family situation that requires him to step away from the shinobi life. For instance, if he has a child with special needs that demands his full attention.
In fanfiction, it could be for safety reasons. There might be threats or political situations in the village that make them keep their marriage a secret. For example, there could be some groups who oppose their union, and by keeping it secret, they can avoid any potential problems until they are strong enough to face them. Also, they might just want to have a simple, intimate bond without all the formalities and public attention that a normal wedding would bring.
One possible plotline could be that Naruto's mother, being an Uchiha, passes on some unique Uchiha abilities to Naruto. Maybe he has an easier time learning certain jutsus related to the Sharingan, even without actually having the Sharingan himself. For example, he could have a natural affinity for fire - style jutsus which are common among the Uchiha.