Well, it's popular because both Naruto and Kakashi are beloved characters. People like to see their relationship explored deeper. Kakashi is a respected and skilled ninja, and when he shows favoritism towards Naruto, it gives Naruto a boost that fans enjoy seeing. It's like a 'chosen one' kind of story within the Naruto universe.
One good 'strong naruto kakashi favoritism' fanfiction could be 'Naruto's Hidden Strength Under Kakashi's Guidance'. In this fic, Naruto is shown to have extraordinary growth under Kakashi's special training and mentorship. Kakashi focuses a great deal on Naruto, believing in his potential more than others, which leads to Naruto achieving great power early on.
Another reason could be that some fans are simply more interested in seeing a super - strong Naruto and in order to make that more plausible, they put down Kakashi. They might think that Kakashi's teaching was not enough to bring out the best in Naruto and by bashing him in fanfiction, they can create a narrative where Naruto reaches his full potential faster and in a more spectacular way.
Well, one that comes to mind might be 'Naruto's Ascent'. In this fanfic, Naruto is made super strong early on and Kakashi is sort of sidelined and criticized for his teaching methods which are shown as ineffective. Another could be 'The New Path of Naruto' where Naruto discovers a new power source and Kakashi's role as a guide is bashed as he fails to recognize Naruto's true potential.
Kakashi might be initially surprised. But then, he would probably use it as a teaching moment for Naruto. He could say something like 'Naruto, stealing is not the way of a ninja.' And then give him a punishment like extra training.
Kakashi would likely be very concerned. He would immediately assess Naruto's injuries. If they were serious, he would use his medical skills to start treating him right away. His first reaction would be to make sure Naruto was stable and out of immediate danger.
Kakashi might immediately offer a comforting presence. He could put his hand on Naruto's shoulder and just be there silently, letting Naruto know he's not alone.
Kakashi may also get angry. Angry at the people or things that caused Naruto to get those scars. This anger could fuel his determination to make Naruto stronger, so that he can better defend himself in the future. He might then intensify Naruto's training, all while having a new - found sense of protectiveness towards Naruto.
At first, Kakashi might try to distance himself from Team 7's possessiveness. He values his independence and doesn't want to feel smothered. However, as he realizes that their actions are coming from a place of deep love and loyalty, he starts to accept it more. He may even start to rely on them a little bit, for example, allowing them to help him with his paperwork or other small tasks that he usually does alone. This shows a new level of trust and closeness between them.