Perhaps he has lost too many people close to him. If his friends, like Annabeth or Grover, were to die in a particularly tragic way, he could be so grief - stricken that he no longer wants to live in a world without them. He might think that death is the only way to escape the pain of their loss.
Another reason could be disillusionment with the gods. If he realizes that the gods are not as just or benevolent as he once thought, and that his efforts in serving them and saving the world are in vain, he might choose to ask for death. He could feel that there is no point in continuing to live in a world ruled by such flawed deities.
Some gods might be indifferent. They are used to mortals' pleas and may see Percy's request as just another desperate cry from a human. For instance, Zeus, who is often portrayed as self - centered, might not care much about Percy's pain and simply ignore the request.
Maybe Percy unknowingly breaks some ancient and very strict laws of the Gods. For example, he might interfere with some sacred rituals or places that are off - limits to mortals, even demigods like him. In the fanfiction world, the Gods could be very unforgiving when it comes to such violations and decide to kill him as a result.
One possible scenario could be that Percy has faced so many traumas and losses in his battles against the monsters and gods' schemes that he simply feels exhausted and wants an end. For example, if he loses his close friends like Annabeth in a particularly brutal way, it might push him to the edge.
One reason could be extreme despair. Maybe he has faced so many hardships and losses that he feels there's no way out. For example, if he has lost many friends in battles against powerful foes and he blames the gods for not protecting them better.
It could be a new, powerful and malevolent god. This god might be jealous of Percy's status as a hero and the favor he has from some of the Olympian gods. So, the new god devises a plan that makes Percy's life so miserable that he asks for death.
Zeus is a possibility. As the king of the gods, he might see Percy's potential and adopt him to have an ally among the demigods. Another could be Poseidon, since Percy is already his son in the original story. It would be an interesting twist to have Poseidon formally adopt Percy in a more divine way.
Maybe he faced an extremely powerful enemy that he couldn't overcome. It could be a new and terrifying monster created by the Titans or some other dark force.
One possible reason could be that Naruto, coming from his own world with its own power system, might view the gods in Percy Jackson's world as too powerful and controlling. He's used to relying on his own ninja skills and the power of friendship, and the idea of these all - powerful gods might seem unfair to him.
One reason could be that the writer wants to create a more angsty and dramatic story. Sally's death would be a huge shock to Percy and the readers. Another reason might be to test Percy's character, to see how he would handle such a great loss.
After Percy's death, his friends might start a rebellion against the Gods. They would be angry and feel that Percy was unjustly killed. This could lead to a full - scale war between the demigods and the Gods.