It could be for strategic reasons. In the world of ninjas, showing too much emotion can be a weakness. Naruto might think that by sealing his emotions, he can become a more effective ninja and better protect his village.
One possible reason could be to gain more power. Maybe he thought that emotions were holding him back from reaching his full potential as a ninja. He might have seen other powerful ninjas who seemed to be emotionless and thought that was the way to go.
It could also be a part of his training or self - discovery. Naruto might have realized that he was relying too much on his chakra and wanted to find a new way to be strong. By sealing off his chakra, he could focus on other aspects like his physical strength, strategic thinking, or even his ninja skills without the use of chakra.
One possible reason could be to protect himself from the pain and loss he has endured. After all he's been through, like the death of his parents, Sirius, and so many others, turning off his emotions might seem like a way to avoid further hurt.
In fanfiction, the seal on Naruto's heart could imply a hidden power source. It might be something that restricts his abilities until he unlocks it, adding an element of mystery and potential for growth. For example, it could be a seal placed by an ancient clan to protect a powerful chakra reserve that only Naruto can access when he overcomes certain trials.
One possible plot could be that Naruto is put under a powerful genjutsu. This genjutsu messes with his mind and slowly drains his emotions. As a result, he becomes detached from his friends and his sense of purpose. He no longer shows excitement for training or fighting, and his usual determination fades away.
One possible reason could be a misinterpretation of family dynamics. Maybe in the fanfiction, the author portrays Naruto's family in a way that he sees as unfair or unloving. For example, if they are shown as neglecting him or putting too much pressure on him.
Another reason could be that Naruto is lured away by an external force. Some powerful figure or group might convince him that leaving his parents is for the greater good, perhaps in the name of protecting them or the village in a more effective way.
In the fanfiction, it might be that Naruto had different ideals and goals from his family. His strong will to pursue his own path of becoming Hokage, for example, could clash with the family's expectations, causing him to reject them.
Perhaps Naruto has some unresolved issues from his past that he doesn't want to burden his family with. He may think that leaving is the best way to deal with these issues on his own, without putting his family in the middle of his personal turmoil. He might go off on a solo adventure to find himself and come to terms with his inner demons.
One possible reason could be that she was forced to betray him. Maybe an enemy got to her family and threatened her to turn on Naruto. For example, in the Naruto universe, there are many powerful villains who would stoop to such tactics.