His other friends like Shikamaru would try to analyze the situation. Shikamaru, being the intelligent one, would look for patterns in Naruto's 'insane' behavior. He might suspect it's a ruse, but he would also consider the possibility that Naruto is actually in trouble. So he would try to approach Naruto carefully and talk to him to find out the truth.
Another plot idea is that Naruto pretends to be insane to protect someone. For example, if there is a powerful enemy after a friend of his, he could act crazy and claim that he has some sort of prophetic madness that warns him of danger. This would make the enemy focus on him instead of his friend, while he figures out a way to defeat the foe.
Perhaps in the fanfiction, some sort of dark magic or curse has been placed on Naruto. This could be messing with his mind and causing him to act in an insane manner. It could be a way for the author to introduce a new and dark plot element to the story. Another cause might be that he has been betrayed by those he trusts the most, which shatters his mental state.
Madara Uchiha could also be a likely cause. If he somehow managed to return or had a long - planned scheme that involved breaking Naruto's sanity. He was a very powerful and devious foe who could use his knowledge of the ninja world and its powers to target Naruto's mental stability.
One element that makes it funny and insane is when the author takes a normal Naruto concept and blows it way out of proportion. For instance, if the concept of chakra is turned into something like 'chakra candy' that ninjas have to eat to gain power. And when the fanfiction has a lot of physical comedy, like Naruto constantly tripping over his own feet while trying to perform a super - important jutsu. Also, when there are crossovers with really unexpected universes, like Naruto meeting the characters from a Looney Tunes world. This creates a really wacky and fun dynamic.
In this type of fanfiction, the story could also focus on how the village might react differently to Naruto. Without Kushina's presence, which could potentially have been a positive influence, Naruto might face more hostility. His struggle to fit in and gain acceptance could be more arduous, and this could be what drives him to the brink of insanity. Maybe he would develop a more jaded view of the world and the people in it.
Well, there could be a 'naruto insane naruto fanfiction' where Naruto teams up with his enemies in a very unexpected way. Maybe he forms an alliance with Orochimaru not for power, but to save the entire ninja world from a new, greater threat. They would have to put aside their differences and work together, which would be a really insane concept considering their past relationship.
In some cases, it could be a new and powerful enemy. This enemy might have the ability to manipulate chakra in a way that affects Naruto's mind. For example, they could send out waves of negative chakra that slowly erode Naruto's mental state. Over time, these attacks make Naruto go insane as he struggles to fight against this unseen and insidious force.
The alternate storylines are also really cool. Maybe Naruto becomes a villain or there are completely different power - ups and battles. It gives a fresh take on the Naruto universe. Some fanfictions might even introduce new characters that blend well with the existing ones.
One example could be a fanfiction where Naruto is neglected by his friends in the Hidden Leaf Village after a major event, and he starts to develop some rather extreme and uncharacteristic behaviors as a result. Maybe he becomes overly distrustful and starts to isolate himself, which is a far cry from his normal, positive self.