What are some possible reasons for Naruto being sad in fanfic?

Poison or some sort of harmful jutsu effect. Maybe an enemy uses a special poison or a jutsu that slowly weakens Naruto's body. In the fanfic, this could be a new and unique type of attack that he has not encountered before, and his body is not able to withstand it, resulting in fainting.

Perhaps he thinks fanfic undermines the integrity of the original Naruto story. The official story has a certain narrative and character development, and fanfic can deviate from that significantly. When it comes to his connection with Naruto, any deviation in fanfic might be seen as a negative. For example, if a fanfic changes the way he mentors Naruto, it could be something he wouldn't like.

In a more comical sense, Naruto could be really hungry and he thinks Ino has some delicious food. So he breaks in, not realizing it's inappropriate. But this could then lead to a funny adventure where they end up cooking together or something like that.

In some fanfics, there could be a misunderstanding. Suppose Kushina did something that Naruto misinterpreted as harmful to him or his friends. This misinterpretation could lead to his hatred. For example, if Kushina was involved in a mission that Naruto thought put the village at risk, but in reality, she had no choice.

In fanfics where Naruto attempts suicide, it could be due to extreme loneliness. He has often faced isolation in the original story, and in some fan interpretations, that feeling could be amplified to a point where he sees no way out. Another reason might be failure. If he fails in protecting his friends or achieving his goals in the fanfic's alternate storyline, he could feel so despaired that he contemplates suicide.

One possible reason could be that Naruto has faced too many betrayals. Maybe his friends turned on him in the fanfic, which made him lose faith and stop caring.

It could also be that Naruto is forced to leave Sakura due to some external threat or danger. For instance, if there is an enemy targeting Naruto and he fears that being around Sakura will put her in harm's way, he would choose to leave to keep her safe. This is in line with Naruto's selfless nature as seen throughout the series.

One reason could be that the fanfic writer wants to deviate from the original narrative where Naruto gradually gains acceptance. They might want to put him in a situation where he has to struggle more with being unwanted to make the story more interesting. For instance, they could write a scenario where new characters are introduced and they shun Naruto for some reason.

One possible reason could be a misunderstanding. Maybe Naruto misinterprets Hinata's actions or words and distances himself from her. For example, if Hinata is trying to be helpful in a certain situation but Naruto takes it the wrong way. Another reason might be external influence. If there are other characters who try to drive a wedge between them, like some jealous rivals who spread false rumors about Hinata to Naruto. Also, Naruto's own insecurities could play a part. He might feel that he's not good enough for Hinata and decides to let her go on his own accord.

One possible reason could be that he is influenced by Orochimaru's techniques. Maybe he discovers some of Orochimaru's old research and figures out how to use snake summoning for his own good. Another reason might be that he is trying to find a new power source to defeat a very powerful enemy that normal jutsu can't handle.