Well, it could be due to the weight of expectations. Naruto is often seen as the 'chosen one' type of character. In fanfic, Sasuke might feel pressured by this. He might think that no matter what he does, he'll always be in Naruto's shadow. Also, their different upbringings play a role. Naruto grew up alone but with a positive attitude, while Sasuke had a tragic family past. In fanfic, seeing Naruto's strength despite his hardships could make Sasuke question his own strength and lead to depression.
One reason could be to create a lot of angst. If Sasuke cheats, it would deeply hurt Naruto and cause a rift in their relationship, which could lead to a lot of emotional turmoil for both characters. This would make for a very dramatic story.
A moment of desperation. For example, if one of them is about to die in the alley due to an injury or a curse. The other, in a rush of emotion, kisses them as a way to convey all the unsaid things, all the love and friendship that has been there all along.
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.
Some fanfic authors might write it as a form of wish - fulfillment. They might prefer the Naruto - Sakura relationship more than the Sasuke - Sakura one. So, they create a scenario where Sakura turns to Naruto in a romantic way. Additionally, it could be a way to explore the characters' personalities further. For example, how Sasuke would handle being betrayed and how Naruto would deal with being involved in such a situation. It gives a chance to dig deeper into their emotional and moral aspects.
In such a fanfic, one possible reason could be Naruto's feeling of being left out. Maybe he sees Sakura's constant focus on Sasuke and feels neglected in their friendship triangle. Another reason might be jealousy. If Naruto has unrequited feelings for Sakura, seeing her love for Sasuke could turn to hatred over time.
Another possibility is that Naruto was trying to protect someone else from Sasuke. Suppose Sasuke was on a path that would endanger another important character in the story, like Sakura. Naruto, in his eagerness to safeguard Sakura, might have accidentally hurt Sasuke while trying to stop him from taking any harmful actions.
One reason could be the constant threat to the world in the Dragon Ball universe. She might feel helpless as a non - fighter, seeing her friends and family in danger all the time. For example, when powerful villains like Frieza or Cell were on the loose, she could be depressed about her inability to contribute more directly to the fight.
One possible reason could be his thirst for power. He might see Naruto as an obstacle in his pursuit of greater strength. Maybe he believes that by eliminating Naruto, he can prove himself to be the stronger one and gain more recognition in the ninja world.
One reason could be that Gaara, being a powerful and complex character, might have a different view on the concept of friendship. If he were to tell Naruto that his friends are only with him out of obligation or because of his status as a hero, Naruto might start to doubt his relationships and become depressed.
One reason could be to create shock value. By having two main characters kill another important character, it grabs the reader's attention. Another could be to explore a very dark and alternate version of the Naruto universe. Maybe the writer wants to show how far characters could be pushed in extreme situations.