A Naruto fanfiction can be sad when it delves into the sacrifices Naruto has to make. He gives up so much of his personal life and relationships for the sake of the village and his friends. If a story highlights how he loses his close friends, like Sasuke in some darker interpretations, it can be extremely sad. Additionally, if it shows Naruto being misunderstood by the people he's trying to protect, that also adds a layer of sadness. For instance, if the villagers still don't trust him despite all his efforts, it can be a very tragic situation for Naruto.
Well, Kurama's sad story in Naruto Shippuden is complex. He was feared by the villagers in the Hidden Leaf because of his power. When he was sealed into Naruto, he was angry and tried to take over Naruto's body multiple times. His past was filled with being hunted and then trapped. It wasn't until he started to understand Naruto's will and dreams that he began to change. Naruto's unwavering determination and kindness towards him slowly melted his cold heart.
One sad story in Naruto is about Sasuke. His entire clan was massacred when he was young. He had to live with this trauma and the thirst for revenge. He left his friends in Konoha, like Naruto who considered him a brother, to seek power from Orochimaru. This led to a long separation and many battles between them.
Unfulfilled dreams can also be a theme. Naruto has always dreamed of becoming Hokage to gain the respect and recognition of the village. If in a fanfiction, something prevents him from achieving this dream, like an unexpected injury or a political conspiracy, he would be extremely sad. He has put so much effort and hope into this dream, and to see it slip away would be a huge blow to him.
There's a fanfiction called 'A Sad Ending for Naruto'. In this story, Naruto fails to save Sasuke in the end, and he has to live with the guilt and the loss. It's really heart - wrenching as it portrays Naruto's pain and despair. And 'Naruto's Solitary Sorrow' is also a sad one, where Naruto feels like he's truly alone even after achieving so much, as he realizes the cost of his journey has been too high in terms of relationships and personal losses.
Betrayal can also be a big theme. Maybe in fanfiction, Naruto is betrayed by someone he thought was a close friend, adding to his already heavy burden of sadness. This could be a new character or an existing one taking a different path in the story.
One aspect of Naruto's sad life in fanfiction could be his loneliness as a child. He was shunned by the villagers, which would have had a deep impact on his psyche. In many fanfictions, this is explored further, perhaps with him having even darker thoughts during those times.
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 popular 'naruto is sad' fanfiction might be the ones where Naruto is left alone in the village after Sasuke leaves again. In these stories, Naruto feels a deep sense of loss and loneliness. He has worked so hard to bring Sasuke back, and when that fails, it shatters his spirit. He might be seen sitting alone on the Hokage monument, looking out over the village that he is supposed to protect but feeling empty inside.