One possible cause could be his sense of failure. Naruto always wants to protect his friends and village, so if he fails in a mission or is unable to save someone, it can cause him great pain.
The loss of important people in his life is another factor. Whether it's the death of his mentors like Jiraiya or the potential endangerment of his friends, these losses and threats can create a deep - seated pain within Naruto.
In some fanfictions, Naruto's pain might manifest through his actions. For example, he could become withdrawn and isolate himself from his friends. This is because Naruto has always been a social person, so any deviation from his normal behavior could be a sign of his internal pain.
One reason could be some sort of special jutsu or power - up. Maybe he got a new form or ability that suppresses pain. For example, it could be related to his Sage Mode or something new entirely.
One common scenario is when Sasuke's actions are more extreme in fanfiction. He might attack Naruto more brutally during their confrontations, not just physically but also emotionally by taunting him about his orphan status or his dreams. Another is when Sakura, instead of growing to understand Naruto, continues to be cold towards him and even blames him for things that are not his fault, like Sasuke leaving.
In many fanfictions, Naruto might use new jutsus or a deeper understanding of his own power. Maybe he taps into a more powerful form of the Nine - Tails' chakra and combines it with his Sage Mode in a unique way to overpower Pain.
He could feel it through a shared chakra link. Since they are related and Minato sealed the Nine - Tails in Naruto, there might be a latent connection that gets activated in the fanfiction, allowing him to sense Naruto's pain on a physical and emotional level.
In fanfiction, when Naruto gets injured by Pain, it could also trigger some new powers or abilities within Naruto. Maybe the injury acts as a catalyst for him to tap into a deeper level of the Nine - Tails' chakra. His injury might also make the other villages more aware of the threat that Pain poses. They could decide to form alliances with Konoha in order to deal with the Akatsuki as a whole. This would change the political landscape in the Naruto world.
He could focus on chakra control training. Since he often has issues with his Nine - Tails chakra going out of control, this would be a logical step.
There could be. Fanfiction covers a vast range of ideas and concepts. Since Naruto has a huge fan base and Pain is a well - known character, it's not impossible for someone to have written a fanfiction where Naruto sings about Pain. However, it might not be a very common or mainstream idea, so you might have to dig a bit deeper in your search.
The 'naruto pain into power fanfiction' could potentially rewrite Naruto's character arc in a unique way. It may show him in situations where he is pushed to his limits by pain, more so than in the original series. This extreme pain could lead to a more profound and perhaps unexpected transformation of his power. For example, he might discover a hidden power within himself that was only unlocked through experiencing a great deal of pain. This could also lead to new plot twists and interactions with characters like Sasuke or Kakashi, as they react to this new, more powerful Naruto who has emerged from his pain.
I would recommend 'Pain's New Beginning'. In this fanfiction, Pain gets a chance to relive his life after a strange encounter. It has some really cool plot twists that keep you hooked. It also explores his relationships with other characters in new ways.