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 failure also appears often. Naruto might fail to protect someone or achieve a goal, and this makes him feel hopeless and sad. It shows his vulnerable side as he blames himself for not being strong enough.
Another theme is unrequited love. In some fanfics, Naruto might have feelings for someone who doesn't feel the same way, which leads to a lot of heartache for him. For example, if he has feelings for a character who is in love with someone else.
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.
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.
Loss is a common theme. It could be the loss of a Pokemon, like in a battle or due to illness. Another theme is separation, such as when a trainer and Pokemon are forced apart.
Loss is a very common theme. It could be the loss of a friendship, like two ponies who were really close but then something comes between them. Another common theme is disappointment, like when a pony's hopes and dreams don't come true.
Betrayal is also a theme. Sometimes he is betrayed by those he trusts, which causes great sadness. For example, when his fellow angels turn against him. And there's the theme of unrequited love. His feelings for certain characters may not be reciprocated, adding to his sorrow. This is often explored in 'castiel sad fanfic' to show the depth of his emotional turmoil.
A prevalent theme is the pain of change. Maybe Naruto changes in a way that Hinata doesn't expect or can't accept. He could become more distant or colder due to his own experiences. Hinata then has to deal with the heartache of seeing the person she loves transform into someone she hardly recognizes, and she struggles to hold on to her feelings while being constantly hurt by the new Naruto.
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.
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.
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.