In fanfiction, it might be a result of a curse or some dark magic. For example, a villain could have cast a spell on him that weakened his heart both physically and emotionally.
Another possibility is that it's a genetic factor that was passed down from his parents. This could add an interesting layer to the story, exploring how his family history is connected to his current condition.
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.
In this fanfic, a possible cause for Naruto's heart attack could be a hidden genetic defect. Maybe it was passed down from his parents or some sort of unknown bloodline trait that suddenly manifested. It could also be that he was exposed to a strange and powerful chakra that his body couldn't handle and it affected his heart negatively, causing it to malfunction and leading to a heart attack.
In a fanfic, one possible cause could be the loss of his loved ones. If he were to lose Sasuke again, this time perhaps permanently, it could send him into a deep depression. He's always been so focused on bringing Sasuke back, and if that hope is completely dashed, it would be a huge blow.
It could also be due to personal relationships. Perhaps some of his closest friends get caught up in their own lives. Like Sasuke might be on a long - term mission and not be there for Naruto, and Sakura might be too busy with her medical studies. So Naruto, who is used to having a support system, suddenly feels alone and neglected.
In the fanfiction, Naruto might have a fever due to overusing his chakra. When he pushes his body to its limits in intense battles or during extensive training, it could strain his body and cause a fever.
One possible cause could be a high - cholesterol diet. If a man eats a lot of fatty and fried foods over time, it can lead to plaque buildup in the arteries, which may trigger a heart attack.
One possible plotline could be Naruto getting transported to the Fairy Tail world and having his powers suppressed. He then has to learn Fairy Tail magic from scratch. Another could be that Naruto is cursed in the Fairy Tail world which makes him weak, and he has to find a way to break the curse while making friends in the guild.
Perhaps in the fanfiction, some sort of dark magic or curse has been placed on Naruto. This could be messing with his mind and causing him to act in an insane manner. It could be a way for the author to introduce a new and dark plot element to the story. Another cause might be that he has been betrayed by those he trusts the most, which shatters his mental state.
Perhaps he was in a battle against a very strong enemy. The enemy might have used a special attack that specifically targeted his eyes, like a genjutsu that damaged his optic nerves when he broke free from it.
Well, in some fanfictions, Naruto's weak heart might make him more vulnerable to emotional attacks. His usual bravado could be a front, and the weak heart could lead to him doubting himself more often. This could slow down his growth as a ninja as he struggles with self - confidence.