It could also be related to his self - preservation in the wizarding society. In a world full of magic and danger, showing too much emotion might be seen as a weakness. Harry, being aware of this, decides to shut out his emotions to appear stronger. He has faced a lot of scrutiny and jealousy from some students and even some adults in the wizarding world. By suppressing his emotions, he hopes to be more respected and less of a target.
In terms of his romantic relationships, if there are any in the fanfiction, it could be a major obstacle. Shutting out emotions means he may not be able to fully open up to a potential partner. This lack of emotional availability can make it difficult for the relationship to develop. For example, if he has feelings for Ginny, his inability to express his emotions might make Ginny feel unwanted or unloved, even though Harry might actually care deeply.
One possible reason could be to protect himself from the pain and loss he has endured. After all he's been through, like the death of his parents, Sirius, and so many others, turning off his emotions might seem like a way to avoid further hurt.
Ron, on the other hand, might feel a bit betrayed. He's always been close to Harry on an emotional level, and seeing Harry turn off his emotions could make him think that Harry doesn't care about their friendship as much anymore. But in the long run, they would probably try to find a way to bring Harry back to his normal self.
Perhaps he was cursed. In the wizarding world, there are many dark curses that could affect a person's mind, and one such curse could target his emotional understanding. Also, it could be due to his isolation. Harry has often been alone or felt alone, and this lack of regular social interaction with a diverse range of people could have made it difficult for him to develop a normal understanding of emotions.
One possible reason could be to explore a different side of the character. By taking away his emotions, the writer can show how much emotions actually contribute to his actions and relationships. It's like a 'what if' scenario.
One possible reason could be that he might feel abandoned. Growing up without them, he could resent the fact that they left him an orphan. He might think that if they were really so great, they would have found a way to survive and be there for him.
Maybe he was in a dangerous duel with a powerful dark wizard. The dark wizard could have cast a spell that was specifically designed to maim or disarm in the most literal sense.
One possible reason could be a powerful spell backfiring on him. In fanfiction, authors often play around with magic going wrong. Another reason might be extreme emotional distress. For example, if he discovers something truly shocking about his past or his friends.
One possible reason could be that he felt he had learned enough magic to face his enemies on his own. He might have thought that the traditional Hogwarts education was too slow or not relevant to his specific needs, like dealing with Voldemort. Another reason could be some sort of conflict with the Hogwarts faculty. Maybe he didn't agree with their methods or decisions and decided he was better off without their rules.
His character development would likely be very different. Instead of growing through emotional experiences like the loss of loved ones or the joy of friendship, he would develop in a more cold, calculated way. Maybe he'd become a master of spells not because he has a passion for magic, but because it's a means to an end.