One possible reason could be the loss of his friends. Naruto deeply values his relationships, and if something were to happen to his friends like Sasuke leaving again or getting seriously injured, it would shatter his happiness. Another reason might be if he fails to protect the village. Naruto has always seen himself as the protector of Konoha, so if he fails in this duty, his sense of purpose and happiness would be lost.
It could be due to a personal failure. Naruto has always strived to become Hokage and bring peace. If he realizes that he can't achieve true peace no matter how hard he tries, or if he makes a big mistake in his leadership as Hokage, like causing a war instead of preventing one, his happiness would disappear. His whole identity is built around being a positive force for change and peace, so any major setbacks in this regard would be devastating.
Another reason might be related to a plot device in the fanfiction. The author could be using the loss of the mask to create a turning point in the story. It might lead to new relationships being formed or old ones being tested. For instance, his friends might react in different ways when they see his face without the mask, and this could drive the story in different directions.
One reason could be betrayal by his friends. For example, if someone he considered a close ally made a deal with his enemies behind his back, it would shatter his trust. Another might be lies from the village leaders he respected. If they hid important information or manipulated him for their own gains, Naruto would start to question who he could trust.
He might become more withdrawn. Without his happiness, Naruto would lose his usual exuberance. He would probably start to isolate himself from others, as he no longer has that positive energy to share. For example, he might stop going to his favorite ramen place and avoid his friends.
If someone were to insult or harm Hinata, who he loves deeply, Naruto could lose his temper. He's very protective of those close to him, and any threat to Hinata would likely trigger his anger. He has a strong sense of justice and loyalty, so seeing someone he cares about being mistreated would be a big deal.
Maybe he was protecting someone in Wave. He's always the self - sacrificing type, so he could have put himself in harm's way to save a villager or a friend.
One possible reason could be an enemy attack. A powerful and malicious foe might target Naruto's family to get back at him, and his wife could unfortunately become a casualty in the battle.
Well, one possible plot could be that Naruto is cursed by an enemy during a battle. This curse takes away his voice. As a result, he has to find new ways to communicate with his friends and allies. Maybe he starts using sign language, which he has to learn quickly. This could lead to some really interesting character development as he struggles to convey his thoughts and emotions without speaking.
One possible plot could be that Naruto is put under a powerful genjutsu. This genjutsu messes with his mind and slowly drains his emotions. As a result, he becomes detached from his friends and his sense of purpose. He no longer shows excitement for training or fighting, and his usual determination fades away.
One possible consequence could be a change in his fighting style. Without his eye, he might have to rely more on his other senses like hearing and smell, similar to how blind ninja in the Naruto universe adapt. For example, he could focus more on using his chakra to sense the movements of his opponents.
Naruto might also go through an identity crisis. His whole life has been centered around being a ninja with chakra and using powerful jutsu. Losing his chakra could make him question who he is and what his purpose is. He would have to redefine himself and find a new way to contribute to the ninja world. This could be a long and difficult process for him.