One possible reason could be that Naruto has had some really bad experiences that made him lose his confidence. For example, if he faced a series of powerful enemies alone and got severely beaten each time, it might make him scared of everyone. Another reason could be some kind of curse or dark magic that affects his mind, making him fear those around him.
In this kind of fanfiction, perhaps Naruto saw some terrifying future through a special ability. In that future, everyone around him turned into a threat to him. This vision could have been so vivid and disturbing that it made him scared of everyone in the present. Also, if there was a sudden change in his own power, like his chakra becoming unstable and uncontrollable, he might fear that others would see him as a danger and react negatively, which in turn makes him scared of them.
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.
It could be due to a personal loss that has hit him really hard. Maybe he lost someone very close to him, like Jiraiya all over again, but this time it has broken him in a way that he shuts everyone out. He might also be under the influence of a new power that he doesn't fully understand yet, and this power is affecting his emotions, making him cold as a side - effect until he learns to control it.
One possible reason could be a false accusation. Maybe someone spread a rumor that Naruto was involved in a heinous act, and without proper investigation, everyone turned against him.
It could also be due to a misunderstanding. Perhaps there is some misinformation spread about Sasuke in the fanfiction, like false rumors that he has a hidden and malevolent plan. Naruto, being someone who values their friendship deeply, would be scared at the thought of Sasuke turning into an enemy because of these untrue stories.
One possible reason could be a powerful genjutsu cast over the village. Maybe an enemy ninja with strong illusionary powers managed to make everyone forget Naruto. It could also be due to some sort of curse. For example, a dark curse was placed on Naruto that made people's memories of him fade away gradually.
One possible reason could be that Naruto has seen or experienced something that makes him distrust or fear those around him. Maybe he has discovered a dark secret about the village or his friends that has shaken his beliefs.
It could also be that Naruto has had some sort of revelation about himself or the ninja world. He might have realized that the relationships he has been in were holding him back in some way. Maybe he wants to focus on self - improvement and growth, and he believes that being alone is the best way to do that. He could be trying to find his true self without the influence of others.
One reason could be Kakashi's Sharingan. It's a very powerful and somewhat spooky ability. Naruto might see it in action in a way that scares him. Maybe Kakashi uses it to subdue an enemy in a really intense way that makes Naruto realize how dangerous Kakashi can be.
Lucius' power within the dark wizarding world could be intimidating to Draco. He has connections, influence, and a certain level of magical ability. Draco might be scared that if he steps out of line, Lucius could use his power against him, or even put him in danger. Also, Lucius' cold and often unforgiving demeanor could make Draco feel constantly on edge in a fanfiction.
One possible reason could be that Kushina has a very strong and intimidating personality. In the Naruto universe, she was known to be a tough kunoichi. Naruto, who might be used to more gentle or reserved people around him, could be scared of her boldness.