One reason could be the overwhelming pressure. Being Hokage means shouldering the safety and well - being of the entire village. Naruto might have realized that he can't bear that kind of stress forever. Another factor could be personal sacrifice. He may have seen how being Hokage has cost others in their personal lives and he doesn't want to go through the same. Also, he might have new dreams or goals that are not compatible with the role of Hokage, like exploring the world outside the village more freely.
Perhaps Naruto realizes that there are others more suitable. He has always been self - deprecating in a way. He might think that someone with more political acumen or better leadership skills in certain areas should take the role. For example, Shikamaru has great strategic thinking which is very important for a leader. Also, Naruto may have grown tired of the constant battles and political intrigue that come with being Hokage. He just wants a simple, peaceful life where he can be with his family and friends without the heavy burden of leading the village.
It could also be that he is tired of the false expectations created by fanfiction. In fanfiction, his character might be portrayed in ways that don't match who he really is. He wants to be true to himself and his ninja way, so he gives up fanfiction.
One possible reason could be that he experiences a great loss, like the death of a dear friend. This might shatter his confidence and make him lose his passion for being a ninja.
One possible reason could be that he experiences a great personal loss. Maybe he loses someone very dear to him during a mission, like his best friend or a respected mentor. This loss makes him question the whole ninja life and all the violence and danger it entails, leading him to give it up.
One possible reason could be the overwhelming stress. Being Hokage means handling a lot of political, social and security issues in the village. Naruto might just feel that he can't take it anymore and decides to quit in the fanfiction. For example, dealing with the constant threats from other villages and internal disputes within Konoha can be mentally and physically exhausting.
With his friends, it might create a sense of shock. His friends who have always supported his dream of being Hokage would be surprised. Some might be disappointed, like Sakura who has seen Naruto's growth towards that goal. But others, like Shikamaru, might understand his decision and still support him in whatever he chooses to do. With his family, if he has a family at this point, it could bring a sense of relief as they may not want him to be in such a dangerous and stressful position.
One possible reason could be a series of misunderstandings. Maybe his family made decisions that he felt were against his principles or dreams, and after repeated attempts to reconcile the differences, he just lost hope. For example, if his family was against his ninja way, which is such a core part of his identity.
One reason could be the constant setbacks he faces. He might feel that no matter how hard he tries, he can't achieve his goal. For example, if he fails too many times in battles against strong opponents, he could lose hope. Another reason might be the loss of important people in his life. If someone like Iruka or Kakashi, who have been his pillars of support, were to die, it could shatter his dreams as they are so closely tied to his will to succeed for their sake.
The Third Hokage was a symbol of stability in the village. Naruto, who grew up without parents and was sometimes an outcast, would see the Third Hokage as a source of strength and guidance. Also, the Third Hokage's knowledge of various jutsus and ninja arts could be something that Naruto admires in fanfiction, as Naruto is always eager to learn new techniques.
One reason could be the sacrifices he has to make for the village. Naruto is so dedicated to protecting Konoha that he might feel he doesn't have time for love. His responsibilities as Hokage are immense, and he has to constantly be on guard against threats. Another reason could be past traumas. He has lost many people he cared about, like Jiraiya. This could make him afraid of getting close to someone again and losing them, so he gives up on love.
One possible reason could be a lack of time. Maybe Ash has become too busy with other aspects of life, like work or studies, and can't find the time to write fanfiction anymore.