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.
Perhaps he has been betrayed in some way, like by someone he trusted deeply who was in league with the gods. He might feel so angry and hurt that he wants to end his life as a form of protest or just to escape the pain.
Perhaps he has been through some sort of mental and emotional breakdown. Maybe he has lost too many battles, seen too many friends die, and he feels that he is a burden or that his death would somehow bring peace to his troubled mind. It could also be related to some prophecy or dark secret he has discovered about himself that makes him think death is the best option for everyone.
Some gods might be indifferent. They might see Percy as just a mortal who has outlived his usefulness. For instance, a god who only cares about power and has no real connection to Percy would simply ignore his request.
One possible reason could be extreme guilt. Maybe Naruto has done something he believes is unforgivable, like accidentally causing the death of a beloved friend while in a fight. He might think that death is the only way to atone for his mistake.
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.
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.
Well, Harry may have realized that Sirius is his godfather, which means there's a special bond already. He might think that living with Sirius would give him a proper family environment, filled with love and understanding, unlike what he has at the Dursleys' place. Also, Sirius has a rebellious and free - spirited nature that Harry might be attracted to. He could hope to grow up in a more exciting and accepting atmosphere.
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.
It might also be due to a sense of hopelessness. If he's faced with an enemy that seems truly unbeatable and he fears the destruction of the village or his friends, he could ask to be killed to prevent further harm. For example, if there's a powerful villain who has Naruto's friends hostage and the only way to save them is by Naruto sacrificing himself.
Maybe Harry has some new information about a greater threat that would also endanger Voldemort. So, he asks Voldemort to save him so that they can join forces against this new threat. Like a new dark entity from another dimension that is coming to destroy the wizarding world, and Harry figures Voldemort wouldn't want to lose his own power base.
Naruto might be longing for a motherly figure in his life. He has always been alone and has seen how others have mothers. Tsunade, being a strong and caring woman in the village, could be someone he looks up to and wishes to have as a mother.