Maybe she has lost her faith or confidence in her ability to heal with chakra. In the story, she might have faced some traumatic event related to her healing attempts that made her subconsciously suppress this power. Like she accidentally made a situation worse while trying to heal, and now she can't bring herself to use chakra for healing again.
One possible reason could be a disruption in her chakra control. If she has some internal blockage or instability in her chakra system, it might prevent her from using it for healing. For example, she could have been affected by a curse or some sort of dark chakra influence that throws off her normal chakra functions.
It might create more conflict. Since she can't heal, the group she is in will face more challenges when they get injured. This can lead to difficult decisions about whether to continue their mission or retreat, adding more tension to the story.
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.
Maybe she has some unresolved issues from her past, like her unrequited love for Sasuke that still lingers in the back of her mind. This could make her question her relationship with Naruto and lead to her leaving.
One possible reason could be a misunderstanding. Maybe she was misled by false information about Naruto being a threat to the village. Another reason might be that she was under some kind of mind control. In some fanfictions, dark magic or evil entities can take over a character's mind and make them act out of character.
One possible plot twist could be that Sakura discovers a hidden power within herself that is even more powerful than her healing chakra. Maybe it's a dormant bloodline limit that gets activated due to the loss of her healing ability.
One possible reason could be that she sacrifices herself to save Naruto from a powerful enemy. In the heat of battle, she might put herself in harm's way to ensure Naruto's survival.
One possible reason could be extreme jealousy. Sakura might be so blindly in love with Sasuke that she sees Naruto as a threat to Sasuke's attention or her relationship with Sasuke. Maybe she thinks Naruto is getting in the way of Sasuke's return to the village or her attempts to win Sasuke back.
It could also be related to Sakura's strength. Sasuke is constantly striving to become stronger. If Sakura somehow shows a weakness or makes a mistake during a fight that he thinks could have been avoided, and it has negative consequences for their situation, he might get angry at her for not being more capable.
Perhaps Sakura is in a situation where she thinks Naruto's actions are endangering the village or their friends. In the Naruto universe, they often face difficult choices and she might believe that Naruto's continued existence is a greater threat than his death at that moment. It could also be that she has been poisoned or affected by some sort of jutsu that clouds her judgment and makes her say such a cruel thing.
In fanfiction, Sasuke might snap at Sakura due to miscommunication. Sakura could be expressing her concern for him in a way that he misinterprets. For instance, if she tells him not to go on a dangerous mission and he thinks she is trying to control him instead of just caring about his safety. Another factor could be the influence of external forces. Maybe there are other characters or events that put him in a bad mood, and Sakura just happens to be there at the wrong time, so he snaps at her.