It could be due to the fact that Kakashi saw the threats Naruto was facing and knew that his earlier training could have better equipped Naruto to deal with them. Naruto had to face powerful enemies quite early, and if Kakashi had trained him from the start, Naruto might have been more confident and capable in combat situations.
Maybe Kakashi regretted it because he saw how much Naruto was struggling without proper guidance. Naruto was often left to his own devices in terms of training in the early days. Kakashi, with his skills and experience, could have provided a more structured and effective training regime which would have saved Naruto a lot of trial and error.
One reason could be that he saw Naruto's potential later on and realized he could have been much stronger if he had trained him earlier. For example, Naruto has a great deal of chakra and tenacity, and with proper training from Kakashi earlier, he might have reached new heights faster.
It might lead to Kakashi taking on a more intensive training role for Naruto later. This could fast - track Naruto's growth in skills and power, changing the power dynamics in the story.
Maybe Kakashi is preoccupied with other important missions. He is a highly - skilled and often - needed ninja. There could be some top - secret missions that require his full attention, leaving him no time to train Naruto properly.
One possible reason could be that Kakashi might think Naruto is not ready yet. Maybe Naruto hasn't mastered some basic skills that are prerequisites for the more advanced training Kakashi has in mind. Another reason could be that Kakashi is worried about Naruto's recklessness. Naruto has a tendency to rush into things without thinking, and Kakashi might fear that this could lead to Naruto getting seriously injured during the training.
Another reason could be that in this fanfiction, the plot is set in a way that there is a new training system in the village. And this system doesn't involve Kakashi in Naruto's training. There could be new mentors or a completely different approach to training young ninjas like Naruto, which excludes Kakashi from the equation.
In the fanfic, Kakashi might neglect Naruto's training due to a variety of reasons. He could be preoccupied with his own research or a secret mission that demands his full attention. There could also be a misunderstanding between him and Naruto. Maybe Naruto has done something that Kakashi misinterprets, causing him to distance himself from Naruto's training. Additionally, the fanfic could introduce new characters or plot elements that draw Kakashi's focus away from Naruto. For example, a new threat to the village that Kakashi feels he must handle alone, thus neglecting his duties as Naruto's trainer.
It could be due to a misunderstanding. Kakashi might think that Naruto is getting involved with something dangerous or has some potential that he can't handle. So, in his warped way of thinking, he abandons Naruto to protect him from himself or from some unknown danger.
In the fanfiction, Kakashi might train Naruto in teamwork. Even though Naruto is often a solo fighter, Kakashi could show him the importance of working with others. He could arrange mock battles where Naruto has to team up with other characters, like Sakura or Shikamaru, and learn how to coordinate their attacks and defenses.
One possible reason could be that Kakashi is preoccupied with other important matters in the village. Maybe he has some top - secret missions or tasks assigned by the Hokage that demand his full attention, leaving him with little time to focus on Naruto.
One possible reason could be that Naruto might feel Kakashi is too strict with his training. Maybe Kakashi's methods are too harsh for Naruto, making him frustrated and gradually developing dislike.
One possible reason could be Kakashi's reputation as a strict and powerful teacher. Naruto might be in awe of his skills and fear making mistakes in front of him.