In the fanfiction, it might be that Sakura abandons Naruto because of unrequited love. She could have been in love with him for a long time, but seeing that he has feelings for someone else or is not able to reciprocate her feelings in the way she wants, she decides to move on and abandon their relationship in the story.
One possible reason could be that Sakura realizes she has different goals or values that are not compatible with being around Naruto. Maybe she discovers a new path in life that doesn't involve him, such as focusing on her own medical ninja career without Naruto's chaotic lifestyle interfering.
Another reason could be related to some external threats or events. For example, there could be a situation where Sakura believes that being around Naruto puts her or others in danger. Perhaps there is a powerful enemy who specifically targets Naruto and those close to him, and Sakura, in an attempt to protect herself or others, chooses to abandon Naruto in the fanfiction.
Maybe she has some unresolved issues from her past, like her unrequited love for Sasuke that still lingers in the back of her mind. This could make her question her relationship with Naruto and lead to her leaving.
Perhaps Kakashi feels that Naruto is becoming too reliant on him and not developing his own skills independently. So, in order to force Naruto to become stronger on his own, he decides to abandon him. This is a tough - love approach. Naruto has great potential, but he needs to learn to stand on his own two feet, and Kakashi may think this is the best way to make that happen.
In some fanfictions, Naruto might abandon his family due to a false prophecy that convinces him he's a danger to them. He may think that by leaving, he can protect them from harm that might come his way because of his past or enemies.
One possible reason could be some sort of dark influence or curse that makes her lose her maternal instincts. Maybe a powerful enemy targets her and in order to protect Naruto from further danger related to her, she feels forced to abandon him. Another reason might be that she is made to believe false information about Naruto, such as that he will bring great disaster to the village in the future.
One possible reason could be that Naruto discovers some dark secrets about the village leadership that go against his values. For example, if he finds out that they have been using underhanded means to maintain power.
One possible reason could be his sense of duty. Sasuke has always been driven by his goals for the village or his personal vendettas. Maybe he feels that his mission is more important than staying with Sakura and their unborn child, believing that his actions are for a greater good that will ultimately protect them as well.
In some fanfiction scenarios, Kurama might abandon Naruto if he believes that Naruto is becoming too corrupted by power. Kurama has a sense of honor in a way, and if Naruto starts to use his power in a way that Kurama doesn't approve of, like becoming overly aggressive or cruel, Kurama could decide to leave. For example, if Naruto starts to use the Nine - Tails' power to dominate others rather than protect them.
It could also be due to external pressure. The media or some powerful figures in the fanfiction world might force Tony to stay away from Peter. They could spread false rumors about their relationship or use their influence to separate them for their own gain.
Another reason might be related to Pikachu's own growth. As Pikachu becomes more powerful and self - aware, it might feel that it needs to go on its own journey without Ash. It could be seeking new challenges and experiences that it believes Ash can't provide, like exploring regions that Ash has no intention of going to or training with Pokémon of a different caliber.
Perhaps the death of a key person in his life could lead to him abandoning his dream. If someone like Iruka or Kakashi, who were important mentors to him, died in a way that made Naruto question the purpose of his dream. He might think that without their support and guidance, his dream was no longer worth pursuing.