Perhaps he discovers some dark secrets within the ninja world. For example, if he finds out that the ninja system is corrupt and is being used for unjust purposes, he may decide to give up. He has always been an idealist, and such a revelation could break his spirit.
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 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.
Another factor could be the nature of his ninja life. It's full of uncertainties and risks. He may worry that if he falls in love, his loved one could become a target for his enemies. Naruto has made many enemies over the years, and he wouldn't want to put someone he loves in danger, so he decides to give up on love.
Maybe Naruto gets tired of always chasing after Sasuke. He has his own life and dreams too. He might think that he has sacrificed enough of his time and energy on Sasuke, and it's time to focus on himself and the village. He could start to believe that Sasuke has made his own choices and he can't keep forcing Sasuke to come back.
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 Naruto realizes Sasuke is too far gone in his quest for revenge. Naruto might see that no matter how much he tries, Sasuke won't change his ways, so he gives up. For example, if Sasuke keeps attacking innocent people in his pursuit of power to defeat Itachi's enemies, Naruto might finally lose hope.
It could also be due to a misunderstanding. Some false information might have spread in the fanfic's world, making Naruto believe that everyone has turned their backs on him. Like, some bad guys framed his friends, and Naruto, being unable to see through the lies at first, gives up on everyone in despair.
Maybe the loss of a very dear Pokémon friend. If something bad happened to a Pokémon he was extremely close to, like it getting seriously injured or lost forever, it could be so devastating that he loses the will to continue his journey and thus gives up.
In some cases, the fanfiction might be a crossover with another universe where the idea of a ninja doesn't fit. So, to make Naruto fit into this new universe, the writer has to take away his ninja identity. This allows for new and interesting interactions between Naruto and characters from the other universe.
One possible reason could be that Naruto realizes Sakura's true feelings for Sasuke are unwavering. After many attempts to win her heart, he might come to the conclusion that he's just chasing after someone who will never love him back. So, he decides to give up and focus on other aspects of his life, like his ninja duties or his friendships with other people in the village.