Maybe he was facing some serious relationship problems and couldn't find a way to solve them. Or he might have changed and his priorities shifted.
One possibility is that he had personal issues he couldn't handle within the marriage. Another reason could be that he met someone else and felt a stronger connection. Sometimes, it could also be because of differences in life goals and values that became too big to overcome.
One reason could be as a form of light - hearted discipline for something minor, like forgetting an anniversary or a chore. Another might be part of a role - playing game within the relationship that they both enjoy.
In some fanfics, Naruto might leave his family because of a dangerous threat approaching them. He could believe that by leaving, he would draw the danger away and keep his family safe.
One possible reason could be that they have grown apart over time. Maybe they no longer share the same interests or values. For example, if one is very career - driven and the other wants a more family - centered life, it can create a rift. Another reason might be financial problems. If there are constant arguments about money, such as overspending or debt, it can put a strain on the relationship. Also, infidelity could be a factor. If one partner has been unfaithful, it often leads to a breakdown in trust, which is crucial for a marriage.
Another possibility is that they leave because of a sense of disillusionment. They could have witnessed some corruption or unfairness within Konoha's leadership or among the ninja community. This makes them lose faith in the village and decide to start anew elsewhere.
One reason could be to seek greater power. In fanfiction, the world outside Konoha might be seen as having more opportunities for growth. They might want to train with different masters or learn unique jutsu not available in Konoha.
Infidelity could be a reason. If one partner cheats on the other, it often shatters the trust in the relationship, and the betrayed partner may decide that they can't continue. Also, communication breakdown. If they can't talk to each other about their problems, small issues can turn into big ones, and eventually, the man might feel that divorce is the only option.
One reason could be that she suspects something unusual in their relationship. Maybe she noticed some strange behavior or closeness between her husband and the man. For example, if they have a lot of private conversations that she is not part of.
One reason could be to escape the lingering dark magic and the constant reminders of their past battles. Britain, especially Hogwarts, has seen so much turmoil that they might feel the need for a fresh start. Another reason could be to explore the wider wizarding world. They've been mostly centered around Britain in the original stories, and in fanfiction, they could leave to discover new magical cultures and spells in other parts of the world.
Maybe he feels ostracized in Konoha. If for some reason the villagers still don't fully accept him despite his heroics, he could be lured by the idea of a fresh start in Iwa.
Well, perhaps he has his own insecurities and fears. Seeing his wife with cancer might make him face his own mortality in a way he can't handle. He might also be influenced by external factors like family pressure or the prospect of starting a 'new' life without the burden of a sick partner. However, these are all rather selfish reasons considering the situation his wife is in.