There are several possible reasons. First, it's a creative way to expand the existing lore. By making Natsu royalty, the author can create new kingdoms, politics, and relationships for him to navigate. Second, it can be used to develop his relationships with other characters in a new way. His friends in Fairy Tail might react differently to him once they find out he's royalty. Finally, it could be a means to explore themes such as destiny and identity. Natsu having a secret royal identity could force him to question who he really is and what his purpose is in the world.
Perhaps the author wants to introduce a sense of mystery. Natsu's secret royalty could be part of a larger mystery in the fanfiction. It could be related to his past, like he was separated from his royal family as a baby for some unknown reason. Another reason might be to explore different power structures. As royalty, Natsu would have different kinds of power and influence compared to just being a dragon slayer in Fairy Tail.
Yes, in some fanfictions Natsu can be secretly royalty. It adds an interesting twist to his character. Maybe he was a lost prince all along, and his dragon slayer powers are somehow related to his royal heritage. This concept can open up a whole new world of storylines, like him reclaiming his throne or having to deal with political intrigue in the magical world.
There are several potential reasons. It could be a result of a psychological experiment gone wrong. Some fanfictions like to play with the idea of Naruto being a test subject, and this could lead to his emotions being numbed. Another aspect could be related to his role as a leader. In some fanfictions, to be a strong and effective leader, he might believe he has to hide his emotions. Additionally, it could be a reaction to the expectations of others. If everyone expects him to be a fearless hero all the time, he might start to suppress his true emotions to meet those expectations.
One possible plotline could be that Natsu has been secretly married to a character from his past. Maybe it was a rushed decision during a dangerous adventure, and they kept it a secret to protect each other. As the story unfolds, his friends start to suspect something is different about him. He becomes more cautious and less reckless, which is out of character for him.
One reason could be for the shock value. Authors might want to create a very different and unexpected scenario to draw readers in. Another could be a misinterpretation of Natsu's passionate and brash nature as something more negative and controlling.
In a lot of these fanfictions, it could be that Harry has a secret mentor. This could be an old wizard who has been secretly guiding Harry in potions, away from the prying eyes of the Hogwarts staff. This mentor could have seen Harry's potential early on and decided to help him develop his skills in private. Another aspect could be that Harry's own determination and his need to prove himself in a different area other than Quidditch or fighting Voldemort drives him to secretly practice and become good at potions. Also, the idea that there are hidden aspects of magic within him, perhaps related to the founders of Hogwarts, that only manifest in his potions - making ability.
One reason could be that they want to avoid the chaos that would ensue among their friends and guildmates. Their relationship might be seen as unexpected, and they might fear the constant teasing and interference. For example, some of their more rambunctious friends could make a big fuss and it would disrupt their normal lives within the guild. Also, they could be worried about how their relationship might affect their standing in the guild as strong mages. They may want to keep their relationship private until they are sure it can withstand any potential problems that might come from being public.
One possible reason could be that he accidentally causes a great deal of destruction during a mission that the guild can't overlook. Maybe he loses control of his dragon slayer magic and wrecks an important building or site, leading to his expulsion as a form of punishment.
One possible reason could be a misunderstanding during a mission. Maybe Erza's actions were misinterpreted by Natsu, leading to him getting hurt, either physically or emotionally.
One reason could be to protect the person he marries. If she's a commoner, for example, the noble society in Camelot might not accept their relationship. So, they keep it a secret. Another reason could be that the person he marries has some sort of power or identity that could put them in danger if it was known. For instance, if she was a sorceress, in Camelot where magic is feared, they'd have to marry secretly.
One reason could be that the fanfictions don't match his view of the Fairy Tail world. He has his own understanding of the guild, his friends, and their adventures, and when fanfictions deviate from that, he doesn't like it.