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.
It could be a result of a dare or a bet. Maybe their friends dared them to kiss in the alley. At first, they might be reluctant, but as they look into each other's eyes, they feel something more than just the dare and the kiss becomes a real, emotional moment between them.
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.
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.
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.
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 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.
Well, I'm sure there are. Fanfics cover a wide range of themes and ideas. Since Sasuke and Naruto have a very complex and deep relationship in the original Naruto series, it's not surprising that some fans would write fanfics where they share a kiss. It could be a way to explore their relationship in a different light, perhaps a more romantic or emotional way that wasn't fully explored in the official story. You can try searching on fanfic platforms like Archive of Our Own or Fanfiction.net to find such fanfics.
One reason could be Sasuke's thirst for power. If he believes Naruto is the only obstacle in his path to ultimate power, he might be driven to kill him. In some fanfictions, Sasuke may be influenced by a dark entity that twists his mind into thinking Naruto is a threat that needs to be eliminated.
One reason could be Naruto's sheer determination. He has always been the one with an unwavering will, and in this fanfic, that might be the key factor. His never - give - up attitude gives him the extra boost to defeat Sasuke.
Maybe Sasuke goes too far in his pursuit of power and tries to harm someone Naruto cares about deeply in the Forest of Death. Naruto, in a fit of rage and to protect the person, kills Sasuke.