One possible reason could be for power enhancement. In some fanfic concepts, drinking blood might be associated with gaining special abilities or strength. For example, if there's a dark magic or forbidden jutsu involved that requires the consumption of blood to unlock certain powers.
Another reason might be related to a survival situation. If he's in a very dire and desperate situation where there's no other source of sustenance, the idea of drinking blood could be introduced as an extreme measure. However, this would be quite a departure from his normal character in the original series.
Perhaps in this fanfiction, Naruto is in a situation where he has no other source of sustenance. He could be trapped in a desolate place with no food or water, and the blood becomes a last - resort option for survival. This would be a very dark and extreme take on the character, as it goes against his usual values.
One reason could be extreme emotional pain. Maybe he witnesses the death of a very close friend or loved one in the fanfiction world. This overwhelming grief could cause such a physical manifestation as crying blood.
Well, one possible plot twist could be that Naruto's coffee - drinking habit reveals a hidden power within him. Maybe the caffeine in the coffee interacts with his chakra in an unexpected way, causing his jutsu to become super - charged.
Another reason might be that he has some internal conflict. For example, he could be doubting the purpose of the training. If he doesn't see a clear goal or if he's starting to question the methods, he might refuse. In some fanfics, he could also be influenced by new friends or ideas that make him question the traditional ninja training regime.
Physically, he might gain new powers or abilities as mentioned before. But there could also be negative effects. His body could reject the foreign blood, leading to internal pain and weakness.
Well, it's a very strange concept. One possibility could be that Naruto gains some sort of quantum - like abilities. Maybe he could exist in multiple states at once, like being both here and there in the ninja world, which would give him an edge in battles as his opponents wouldn't be able to predict his exact location.
Maybe he discovers some dark secrets in the Blood Prison. These secrets could be about the corruption of the ninja world or some hidden evils related to powerful people. This new knowledge might make him lose faith in the system he once believed in, leading to his transformation into a darker version of himself.
Maybe he was poisoned. There are many villains in the Naruto world who use poison. If Naruto was poisoned during a fight or an ambush, it could lead to him coughing blood as his body tries to fight off the toxins. Just like how Sasuke was once poisoned and had to be treated.
One possible reason could be extreme stress or trauma. In the Naruto universe, he has faced many difficult situations. If a fanfic writer wants to show the toll it takes on him, white hair could symbolize that. For example, if he loses someone very close to him like Sasuke in a more permanent way, it might be so devastating that it causes this physical change.
One possible reason could be for comical effect. It might be a humorous take on the idea of putting a powerful wizard like Harry in an unexpected and somewhat demeaning situation. Writers sometimes use such out - of - the - box ideas to add a unique twist to the story and make readers laugh.