One possible reason could be her deep love for Naruto. Despite her actions, she realizes that Naruto is the one she truly wants. Cheating goes against her values and her feelings for him, so she regrets it as she understands the harm it has caused to their relationship.
In the fanfiction, Hinata could regret cheating because she finally understands the unique bond she had with Naruto. Maybe she was lured into the cheating situation by momentary confusion or the wrong influence, but when she comes to her senses, she sees how special Naruto was to her all along and wishes she hadn't strayed.
One reason could be that Naruto finally realizes Hinata's true feelings for him. After years of her being by his side, supporting him silently, and he may have taken her for granted or even been unkind without fully understanding her depth of emotion. Another aspect could be that as he matures, he sees how his actions, perhaps his obliviousness or his focus on other things, have hurt her. It's like when you grow up and look back at how you treated someone who truly cared for you, and you feel ashamed.
In fanfiction, if Hinata is portrayed as cheating, it could be for shock value. Writers sometimes want to create a big stir in the story.
It could be due to miscommunication. Suppose there was a misunderstanding between her and Naruto. For example, Naruto might have said something that Hinata misinterpreted as him losing interest in their relationship. This confusion could have led her to seek comfort in another person, which unfortunately led to cheating.
Naruto might be initially heartbroken. He is a character who values loyalty and love deeply. So, when he discovers Hinata's cheating, he could be in a state of shock and disbelief.
He would probably have to work really hard to repair the relationship. In fanfiction, Naruto's regret might mean he has to face Hinata's pain directly. Since Hinata has always been so devoted to Naruto, his cheating would be a huge shock. Naruto might try to be more open and honest about his feelings in the future. He could also face social consequences in the village. People might gossip about him and look at him differently. To fix things, he would need to show that he has truly changed and is committed to Hinata again.
Hinata might be very cautious at first. She has been deeply hurt by Naruto's cheating, so even though he regrets it, she would need time to heal. She may not immediately forgive him and might keep her distance to protect herself.
One possible reason could be that she realizes she truly loves Naruto too much to be apart from him. Maybe she left because of some misunderstanding or external pressure, but then comes to understand that her place is by his side. Another reason might be that she sees how much Naruto has grown and changed, and regrets not being there to support him during that journey. Also, she could have found that life without Naruto is just not as fulfilling as she thought it would be.
Since it's a fanfiction, the idea of Hinata cheating could be a very unorthodox and out - of - character concept. It might be the result of the author wanting to create a controversial or alternative storyline. But in the official Naruto lore, Hinata is not associated with such behavior.
His regret can create a more equal and loving relationship. Before, Naruto may have been somewhat dismissive of Hinata, but now that he regrets it, he gives her the respect she deserves. He might start to see her as an equal partner rather than someone he can take for granted. This change in his attitude allows Hinata to feel more confident in the relationship, and they can build a deeper emotional connection based on mutual understanding and love.
One reason could be envy. Maybe Naruto sees Hinata as having something he doesn't, like a more stable family situation, and he lashes out in jealousy. Another reason might be that he's just being a typical, rambunctious kid who doesn't realize the impact of his words or actions on Hinata.