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.
Unrequited love is a common theme. Often, characters in the harem have feelings for Naruto that are not reciprocated, leading to sadness. Another theme is sacrifice. Some characters might sacrifice their own happiness for the sake of the group or Naruto, which is really sad.
The theme of hope against all odds is also present. Despite his sad childhood, Naruto in fanfiction always has that glimmer of hope that things will get better. He believes in himself and his future, even when no one else does, and this is something that fanfic authors like to emphasize.
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.
The theme of loneliness can also be prominent. Naruto, despite being surrounded by friends, can feel lonely because the person he loves doesn't love him back. And Sakura might feel lonely in her own pursuit of Sasuke, realizing that Naruto was always there for her but she didn't see it. These themes all contribute to the overall melancholy of Naruto and Sakura's love in fanfiction.
One common theme could be loneliness. Naruto often felt alone during his childhood, and at Christmas, this feeling might be intensified. Another theme could be sacrifice. Characters might sacrifice their own Christmas cheer for the greater good of the village. And also, the theme of unfulfilled wishes, as not all characters can have their ideal Christmas due to the harsh shinobi world they live in.
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 theme of time and change is also prevalent. As time passes, their feelings might change or they might change as people. Naruto could become more distant because of his experiences, or Sakura could become more hardened. This change can lead to a sad love story where the love they once had or could have had is now lost. For instance, Naruto might go through a traumatic event on a mission that changes his view on love and relationships, making it difficult for him and Sakura to be together.
One common theme is unrequited love. Maybe Naruto loves someone else and Temari has to deal with the heartbreak of loving him from afar. Another theme could be separation. Due to various reasons like village duties or misunderstandings, they are forced to be apart. This creates a lot of sadness in the story.
Isolation is a common theme. Naruto was often alone as a child, and fanfics may expand on this, showing his deep - seated feelings of loneliness.
Betrayal is also frequent. Whether it's being betrayed by a friend or someone he loves, it adds a lot of sadness to the story. Like when a trusted ally turns against him because of his magic. It really tugs at the heartstrings.