One reason could be a dangerous magical accident. Maybe he was experimenting with some powerful and unknown magic that went wrong. Another could be due to a dark wizard's plot. The dark wizard might have found a way to bypass all his protections and kill him.
Some fanfiction authors might have Harry die young as a way to explore the consequences and how the wizarding world would change without him. He could be killed in a battle against a new, emerging evil force. His death would then lead to a whole new set of power dynamics and character developments for the remaining characters in the story.
In some fanfictions, Harry could die at a young age because of a deadly disease that has no cure in the wizarding world. It could be something like a magically - enhanced version of a Muggle disease that the wizarding medicine can't handle. Also, it might be related to a sacrifice. For example, to save his friends or the whole wizarding world, he gives up his own life.
One possible reason could be a dark magic curse. In many fanfictions, an ancient or powerful dark magic spell might be placed on Harry, and despite all his skills and the help of his friends, he couldn't overcome it. This could be something that was secretly cast on him during his early years, perhaps related to his connection with Voldemort.
One reason could be a different family situation. For instance, if he was raised by someone other than the Dursleys, who might have been more involved in the dark arts. Another could be his experiences at Hogwarts. If he faced more discrimination or was wrongly accused often, it could lead to a sense of bitterness and a turn towards the dark side.
Perhaps he gets a chance to attend a special magical school or training program that requires him to leave the Dursleys. It could be a place where he can develop his powers without the Dursleys' interference. Also, he may be running away from a threat that the Dursleys are unable or unwilling to protect him from.
One reason could be extreme stress. In fanfiction, Harry might be facing a new and overwhelming threat, like a powerful dark wizard that even his skills can't easily handle. The pressure of having to protect his friends and the wizarding world could cause him to faint.
Another aspect could be the never - ending battle against Voldemort. The fear, the stress, and the constant danger might have taken a toll on his mental health. He might think that dying would end this cycle of fear and uncertainty.
There are several possible reasons. Firstly, for the sake of drama. It makes for a very intense and emotional story when a beloved character like Percy dies. Secondly, it could be to explore the after - effects on the other characters. How would Annabeth cope? How would the Camp Half - Blood react? Also, it might be a way to show the unforgiving nature of Tartarus.
There could also be some sort of genetic or pre - existing condition that the baby has. In the fanfiction world, the mpreg concept may come with its own set of genetic anomalies. If these are not properly addressed or monitored in the nursery, it could lead to the baby's death.
One possible reason could be a powerful new enemy. Maybe a villain with an unknown and extremely deadly jutsu appears. This enemy could be so strong that Naruto, despite all his power, is no match for them. For example, an enemy who can nullify Naruto's chakra or has the ability to manipulate time and space in ways that Naruto can't counter.
One reason could be the loss of his father. In the Naruto universe, Kakashi had a complex relationship with his father's death. If a fanfiction delves deeper into his younger years, this could be a very emotional trigger for him to cry.
One possible reason could be extreme depression. Maybe in the fanfiction's plot, Harry has faced so many losses and hardships that he feels there is no way out. For example, if all his friends turn against him or he fails to achieve his goals repeatedly.