It could also be due to the constant pressure and expectations on him. He's always had to be the hero, the one to save the village. If he feels he's constantly failing or not living up to those expectations, he may close his heart to protect himself from that self - doubt and disappointment. In the fanfic, perhaps there are new threats to the village that he feels he can't handle, and this leads to him shutting down emotionally.
Another reason might be related to a discovery about his past. Maybe he finds out some dark secrets about his family or the origin of his powers that make him feel like an outcast or unworthy. This could cause him to close his heart as he tries to come to terms with this new and disturbing knowledge.
Perhaps after facing too much betrayal from the village he loves. If the villagers turned on him despite all his efforts to protect them, it might lead him to close his heart in self - defense, not wanting to be hurt again. He's always tried so hard to gain their acceptance and if that was all for naught, it would be a huge blow.
Well, it might start with Naruto distancing himself from his friends. He stops going on missions with them, and he doesn't show up at their usual gathering places. For instance, he might avoid Ichiraku Ramen, which was always a place of connection for him and his friends.
One possible reason could be to protect his friends. He might think that by sacrificing his heart, some great danger or enemy would be vanquished, thus ensuring the safety of those he cares about. For example, if there was a powerful curse that could only be broken by his heart's sacrifice.
A 'demon in his heart' in a Naruto fanfic could refer to many things. It might be about a character who has an inner struggle with a 'demon' - perhaps a dark side of their nature, like Naruto's struggle with the Nine - Tails. It could also be about a new character introduced in the fanfic who has a 'demon' within them that affects their actions and relationships with other characters in the Naruto universe.
Self - reflection is key. If Naruto takes the time to really look at himself and understand why he closed his heart in the first place, he can start to work through those issues. Maybe he realizes that closing his heart is only causing him more pain in the long run and that by being open, he can experience the full range of human emotions again, both the good and the bad.
One possible theme could be redemption. If a character has a 'demon in his heart', they might strive to overcome it and find redemption. Another theme could be self - discovery. As the character battles their inner 'demon', they may learn more about themselves.
Maybe he wants to protect his loved ones. If his enemies know his true strength, they might target his friends and family to get to him. So, by hiding his skills, he can keep them out of harm's way while still being able to fight when necessary.
In fanfiction, the seal on Naruto's heart could imply a hidden power source. It might be something that restricts his abilities until he unlocks it, adding an element of mystery and potential for growth. For example, it could be a seal placed by an ancient clan to protect a powerful chakra reserve that only Naruto can access when he overcomes certain trials.
Perhaps in the fanfic, Naruto is trying to fit into a different group or culture. His original hair style might make him stand out in a negative way in that context, so he changes it to be more accepted. Or it could be part of a bet or a challenge. For instance, he loses a bet with his friends and as a consequence, he has to change his hair style.
In some fanfics, Naruto might leave his family because of a dangerous threat approaching them. He could believe that by leaving, he would draw the danger away and keep his family safe.