Perhaps there is a family - related magical ailment that has been passed down. Since his parents were involved in the wizarding war, they could have been exposed to some dark magic that affected their genes in a way that is now causing Harry to be sick at such a young age. Or it could be something as simple as a magical virus that young wizards are more susceptible to.
It could be due to neglect. If we consider the situation in the Dursleys' household, they might not be taking proper care of him. They could be depriving him of proper nutrition or warmth, which could lead to him getting sick easily. Also, he might have been exposed to some dark magic residue in the environment that is affecting his health.
One possible reason could be a magical curse placed on him. Maybe some dark wizard who hated his parents decided to target him from a very young age. Another reason might be related to his unique magical nature. Since he is so powerful, his magic might be unstable at such a young age and could be making him sick as his body tries to adjust.
Sirius, if he were aware of Harry's situation, would be extremely worried and desperate to get to Harry. He would feel responsible for protecting Harry, just like he felt towards James. He might even try to break any magical barriers or rules to get to Harry and take him to a safe place where he could be cured. And Hagrid would be heartbroken. He would bring little gifts like a small magical creature or some warm clothes to try to make Harry feel better.
In a Naruto sick fanfic, one possible reason for his illness could be a curse. Maybe he was targeted by an enemy ninja who cast a powerful and slow - acting curse on him. This curse could be sapping his chakra and making him physically weak.
One possible reason could be a magical curse. In the world of Merlin, there are many dark sorcerers who might target Arthur. Another reason could be a poisoned weapon. If he was injured during a battle, the poison on the blade could make him sick. Also, it might be a result of some ancient plague that has resurfaced in Camelot.
One possible reason could be that Harry made a dangerous decision without consulting Sirius. For example, if Harry decided to face a powerful dark wizard alone, Sirius, being his godfather and concerned for his safety, would be angry.
One possible reason could be a dark magic spell gone wrong. Maybe some wizard tried to experiment with a time - altering spell and accidentally reversed whatever transformation Voldemort (Riddle) had undergone. Another reason might be related to some ancient magic artifact. If Riddle's essence was somehow bound to an object, and that object was tampered with, it could bring him back to his old self.
It could also be psychological. Maybe the child has experienced some sort of trauma that has regressed them to a state where they feel more secure in diapers. Stress or major life changes can sometimes cause such behavior.
He might hide it because he doesn't fully understand his own fascination. Harry has always been on the side of good, but perhaps there are elements of the dark side that seem alluring in a strange way. He could be afraid that exploring these feelings would lead him down a dangerous path, so he keeps it a secret.
One possible reason could be that in the fanfic, the author might twist the story to make it seem like his parents' actions had unforeseen consequences that affected Harry negatively. For example, their fight against Voldemort led to Harry being an orphan and having to bear a great burden from a young age.
One possible reason could be that the owl got lost on its way. Maybe it was attacked by some magical creature or got distracted by a powerful spell. Another reason might be that someone deliberately intercepted the letter. In the wizarding world, there could be dark forces at work who don't want Harry to go to Hogwarts. It could also be a bureaucratic mix - up at Hogwarts itself. Maybe the admissions office made an error in their records and didn't send the letter.
It could be due to a new identity he wants to create for himself in a different magical community. Rial might be a name that is more acceptable or respected in that particular community. He may have fallen in love with the culture or values associated with that name and decided to make it his own.