One possible reason could be unrequited love. If in the fanfiction, Sakura continuously rejects Naruto or shows no interest in him, it can lead to his depression. Naruto has deep feelings for Sakura for a long time, and constant rejection can be soul - crushing.
One possible reason could be the weight of his past. Naruto grew up as an orphan, constantly shunned by the villagers. In the fanfiction, this feeling of isolation might be intensified, making him constantly depressed. Another reason could be the pressure of his goals. He aims to become Hokage but might face numerous setbacks that make him feel inadequate and depressed.
It could also be related to his unfulfilled dreams. Naruto has big dreams for the village and for peace. But as he progresses, he might realize that achieving true peace is far more difficult than he thought. The gap between his ideals and the reality might be leading to his secret depression.
It shows his vulnerability. Usually, Naruto is seen as a strong and determined character. But in this fanfiction, his depression over Sakura reveals that he has a soft side, especially when it comes to love. This adds a new layer to his character.
Perhaps Sakura is overly critical of Naruto in the fanfiction. She might focus on his flaws and not give him any credit for his achievements. Naruto, who has always been striving for recognition, especially from Sakura, would be deeply affected by this. Her constant criticism could chip away at his self - esteem and cause him to sink into depression.
Another plotline could be that Sakura makes a decision that deeply affects Naruto's life in a negative way. Maybe she chooses to side with an enemy against Naruto's wishes, and this betrayal makes Naruto feel so hopeless that he becomes depressed. In these fanfictions, the writers often explore the complex relationships between the characters and how their actions can have far - reaching emotional consequences on others. There's also a possibility that Sakura's actions cause Naruto to lose his dream or his sense of purpose, pushing him into a depressive state as he feels lost without that driving force in his life.
Well, it might be that the fanfiction focuses too much on Sakura in a way that Naruto feels is unfair to the other characters. Naruto is part of a big world with many important people, and if the fanfic just hones in on Sakura all the time, he could get fed up. Also, if the fanfiction has Sakura constantly putting Naruto down or not appreciating his efforts like in the real story, that could be a reason.
In some fanfictions, Sakura may make decisions without considering Naruto's opinion. Naruto values his friends and their input, so if Sakura goes ahead and does something like endangering the team or making a big decision on her own that affects everyone, Naruto could get mad. Also, if Sakura were to insult Naruto's dream of becoming Hokage, that would definitely be a reason for his anger as that dream is very important to him.
In many of these fanfictions, Sakura's depression can strain her relationship with Naruto. If she's depressed because of her feelings for him, she might push him away. For example, she could avoid him or be short - tempered when he tries to be friendly. With her friends like Ino, her depression might make her seem distant, causing Ino to be worried and try to reach out more.
One possible reason could be the loss of his friends. In the Naruto universe, his friends are his most precious. If he were to lose some of them in a really tragic way, like Sasuke turning truly evil and being killed while trying to destroy the village, it could plunge him into severe depression. Another reason might be the weight of his unfulfilled dreams. Naruto has always dreamed of a world of true peace, but if in fanfiction he constantly faces setbacks in achieving this, like new wars breaking out despite his efforts, it could lead to depression.
One possible reason could be the intense and often dark nature of Anbu missions. Naruto might be exposed to a lot of death and destruction, which could lead to his depression. For example, he could witness the killing of innocent people during a mission and feel powerless to stop it.