Well, it can add a lot of drama to the plot. For starters, it could lead to a reconciliation arc between Hinata and Naruto. Their relationship would be in a state of flux, with Naruto perhaps being initially hesitant to forgive or trust her again. This would create tension and anticipation for the readers. Also, Hinata's regret might make her more determined to make amends, which could lead to her taking bold actions to get back into Naruto's good graces and the story's main events.
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.
One reason could be the motherly love. Kushina always loved Naruto deep down. Leaving him means missing out on his growth and important moments in his life. Another reason might be realizing the hardships Naruto faced without her presence. She probably feels guilty for not being there to protect him from the loneliness and the many challenges he had to endure in the village.
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.
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.
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.
Well, perhaps in this fanfiction, Hinata is forced into a situation by some outside influence, like a villain's mind - control, which makes her act in a way that causes Naruto to cry. After she breaks free from the control, she is filled with regret. She then decides to go on a dangerous mission to retrieve something important to Naruto that was lost during the incident as a way to earn his forgiveness.
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.
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 develop greater courage as a result. Since she regrets making Naruto cry, she wants to prove herself worthy of his friendship again. This could lead her to take on challenges she previously would have shied away from. For instance, she could volunteer to lead a difficult mission or stand up to a bully in the village, all while thinking about how she can better herself and regain Naruto's trust.