It could be that Sakura has done something that has disappointed Naruto deeply. This could range from her making a decision in a mission that Naruto didn't agree with, to her not being there for him during a difficult time. These actions might have gradually eroded his love for her.
Well, in the fanfiction context, Naruto may not love Sakura because he has unrequited feelings for someone else. Maybe he has a deep connection with another character that makes him view Sakura just as a teammate. Also, their personalities might not be as compatible in a romantic sense as one would initially think. Naruto is often impulsive and headstrong, while Sakura can be a bit more reserved in some ways, and this could lead to a lack of romantic attraction.
They could become more like colleagues or just friends from the past. There's no longer the romantic tension between them. Sakura might have to come to terms with the fact that Naruto's feelings have changed and try to move on as well. Naruto, on the other hand, may focus more on his other relationships, like with his new friends or teammates.
Perhaps Sakura has grown and changed herself. She might have developed new interests and values that are no longer compatible with Sasuke's. In the fanfiction, she could have met new people who showed her a different kind of love and respect, making her question her feelings for Sasuke.
It could also be due to Sakura constantly doubting Naruto's abilities in the story. Naruto has always been looked down upon by many in the original series, and if in the fanfiction Sakura continuously expresses her lack of faith in him, it could lead to his unforgiveness. For example, she might have refused to believe in Naruto's new jutsu or his plans to save the village, which made Naruto feel really disappointed and unable to forgive her.
Maybe he discovers other dreams or goals. He could find that there are other ways to protect the village and his friends that don't require him to be Hokage. He might be more interested in exploring the ninja world, learning new jutsus, or even teaching the younger generation about ninjutsu rather than taking on the political and leadership role of Hokage.
One reason could be that Sakura is confused about her own feelings. Maybe she has feelings for someone else and doesn't know how to handle Naruto's obvious affection towards her, so she lashes out and ends up hurting him.
Perhaps in the fanfiction, Naruto has had some bad experiences with Sakura. Maybe she has let him down in crucial situations, like not believing in him when he needed it most. This could lead to his feelings of despise as he feels betrayed by her lack of faith.
One possible reason could be that Naruto has seen the dark side of the shinobi world. He might have witnessed too much death and destruction in battles, and this has made him disillusioned. For example, if he lost many friends in a major war, he could start to question the whole concept of being a shinobi.
One possible reason could be that Naruto has grown so much on his own journey. He has faced numerous challenges and made many friends who have become like family to him. So, in a sense, he feels self - sufficient and no longer feels the acute need for his absent parents.
It changes the dynamics a great deal. Instead of the typical unrequited love from Naruto towards Sakura in the original, it flips the script. This makes their interactions less about Naruto pining for Sakura and more about a platonic friendship. They might interact more casually and without the underlying tension of unspoken love.