Loss is a very common theme. This could be the loss of a friend, like Sasuke leaving Naruto. It creates a deep sense of sadness as Naruto has to deal with the emptiness.
The strong emotional connection we have with the characters. We've seen Naruto grow up, so when a fanfic puts him through a sad situation like losing his dream or a loved one, it hits hard.
A sad Naruto fanfic could also revolve around Naruto's relationship with his parents. For example, if in the fanfic, he discovers some dark secrets about his father's past actions that make him question his own identity. This would lead to an internal conflict and a lot of emotional turmoil for Naruto.
The combination of Naruto's story and a sad sonata element makes it special. Naruto has so many emotional arcs, and a sad sonata can amplify those feelings.
One 'naruto sad fanfic' that stands out is 'The Lost Promise'. It focuses on Naruto's feelings of loss and loneliness after Sasuke's departure in a very heart - wrenching way. The descriptions of Naruto's inner turmoil are really touching.
In some fanfics, Naruto may be sad because of unrequited love. He has strong feelings for someone, but that person doesn't return his feelings. Naruto is a very emotional character, and love is an area where he can be easily hurt. So this could be a source of his sadness in the fanfic.
One 'sad sonata naruto fanfic' could be about Naruto's inner turmoil during his darkest moments. Maybe it focuses on his loneliness when the village first shunned him, and the fic uses the concept of a sad sonata to enhance the melancholic mood.
One 'naruto sad life fanfic' could be about Naruto's loneliness during his childhood. In the village, he was shunned by many because of the Nine - Tails inside him. He had to grow up all alone in a small, shabby apartment, longing for acceptance.
A bittersweet ending could involve a sacrifice that leads to a new beginning. For instance, Hinata sacrifices her chance at a romantic relationship with Naruto to save his life. Naruto realizes her importance too late, but Hinata is at peace with her decision. In the end, Naruto honors her by living a great life, and Hinata is remembered fondly.