One reason could be that Gaara, being a powerful and complex character, might have a different view on the concept of friendship. If he were to tell Naruto that his friends are only with him out of obligation or because of his status as a hero, Naruto might start to doubt his relationships and become depressed.
Maybe in the fanfic, Gaara has some secret knowledge about Naruto's future. He reveals this knowledge in a way that makes Naruto feel trapped and helpless. For example, if he tells Naruto that no matter what he does, he will ultimately fail in achieving his long - term goals, this would surely depress Naruto. It's like taking away his hope and motivation, which are two very important things for Naruto.
One reason could be friendship. Gaara and Naruto have a special connection in the original story. In fanfiction, this connection could be deepened, and Gaara would save Naruto because he values their friendship. Just like Naruto once reached out to Gaara, Gaara would do the same for Naruto when he's in danger.
One reason could be that he has deep feelings for her. He might not be used to having such strong emotions for someone and this possessiveness is his way of showing how much he cares.
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.
In fanfiction, Naruto could be depressed after the war because he realizes that the world he fought for is still full of problems. The shinobi system might still be corrupt in some ways, and he may feel that all his efforts haven't really changed the fundamental issues. He might also be suffering from post - war stress disorder, constantly reliving the battles and the horrors he witnessed.
One reason is their friendship. Naruto was one of the first people to see Gaara as a person, not a monster. So, Gaara would feel indebted to Naruto and be willing to risk his life. Another reason could be for the sake of the ninja world. Naruto is often seen as a key figure in maintaining peace, and Gaara, as a responsible leader, would understand the importance of saving him.
In some fanfictions, Gaara might use his past experiences and his once cold - hearted nature to make Naruto feel powerless. For example, if Gaara constantly flaunts his strength which he once used for destruction and Naruto feels he can't reach that level of power to protect his friends, it could lead to Naruto feeling depressed.
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.
Well, in fanfiction, one reason could be the loss of his friends. Percy has been through so many battles, and if he loses someone close like Annabeth or Grover, it could send him into a deep depression. Another reason might be the burden of his prophecies. He's constantly expected to save the world, and that kind of pressure can wear a person down over time.
One reason could be his sense of isolation. In many fanfictions, Merlin might feel alone as he has to keep his magic a secret from most people around him. This constant hiding and fear of discovery can lead to depression.
It could be the influence of a powerful enemy or a dark power. Suppose there is an enemy who has the ability to manipulate emotions or a dark power that corrupts Naruto's mind. This could lead him to change his personality and become more like Gaara. For example, if the enemy makes Naruto feel alone and unloved, he may start to develop the same cold and distant behavior that Gaara once had.