It could be due to the after - effects of the sealing on his internal energy balance. Naruto's body is used to a certain way of handling chakra, and the sealing might have disrupted that. His immune system, which is probably linked to his chakra in some way in the Naruto universe, could have been affected. For example, if the sealing was to contain a powerful and malevolent force, some of that 'bad energy' could have seeped into Naruto's own energy system, causing an imbalance and resulting in sickness.
One possible reason could be the strain on his body from the sealing process. Sealing powerful entities often takes a toll on the user's chakra and physical health. Maybe his chakra system was disrupted during the sealing, leading to internal imbalances that manifested as sickness.
One reason could be that the fanfiction often portrays the characters in a very different way from how he knows them. For example, if it changes their personalities too much, it might make Naruto feel like it's not the same team he belongs to. Another reason could be the overuse of certain plot devices in the fanfiction. Maybe every story he reads has the same cliché situations, like the team always getting into the same type of trouble in a very predictable way. Also, if the fanfiction focuses too much on relationships that Naruto doesn't see as important within the team, it could put him off.
If Naruto gets sick after the sealing, it can slow down the progress of the story. His allies might have to focus on taking care of him instead of moving forward with their plans. This gives the enemies more time to prepare or carry out their own evil schemes.
In some fanfictions, the cause could be a curse. There could be an old enemy of the village or someone with a vendetta against Naruto who placed a curse on him. This curse could be slowly draining his life force, causing him to become sick. Or, it could be related to the tailed beast inside him. Maybe there was some imbalance in the chakra of the Nine - Tails that was affecting Naruto's health. It's also possible that he caught a strange and rare disease from an unknown source during one of his travels to a far - off land.
Naruto's sickness could be a major turning point in the story. It could change his perspective on his powers and his role in the ninja world. He might realize that he needs to be more careful with his use of sealing techniques in the future. This realization could lead him to seek out different types of training or knowledge, which would then shift the direction of the plot towards a more self - reflective and strategic Naruto.
In some fanfictions, Naruto might face weakened chakra control after getting sick post - sealing. This could lead to him struggling with basic jutsu at first. His friends might have to step in to protect him more often until he recovers fully.
Maybe he's tired of the same old dynamics. In fanfiction, sometimes the relationships in Team 7 can be written in a very repetitive way. For example, Sasuke's brooding and Sakura's one - sided love for Sasuke can become overdone, making Naruto feel sick of it all. Also, if the fanfiction focuses too much on the team's conflicts without any real development, it could be a reason.
Maybe Naruto is sick of Sakura in fanfiction because her character is often portrayed as too pushy or one - dimensional. In some fanfictions, Sakura might be overly obsessed with Sasuke and Naruto just gets tired of her constantly going on about it.
Well, in the fanfiction, perhaps Naruto realizes that Konoha's politics are too complex. There are always hidden agendas and power plays. He, being a straightforward person, might get tired of all the behind - the - scenes stuff. Also, he may feel that his friends are changing because of Konoha's influence. For example, they might be more focused on the village's reputation rather than their true feelings, which makes Naruto sick of it all.
In fanfiction, Naruto could feel sick due to overusing his chakra. He often pushes himself to the limit in battles and training, and this excessive chakra use might take a toll on his body, making him feel ill.