One possible reason could be a shift in their loyalties. Maybe they discover some dark secrets about Harry or the side he represents that make them uncomfortable. For example, if they find out that Harry has some hidden motives that go against their own beliefs or values.
One possible reason could be to escape the danger and chaos that might be brewing in England within the wizarding world. Maybe there are dark forces targeting them specifically, and leaving is their only option for safety.
One possible reason could be a misunderstanding. Maybe they misinterpreted something Hermione said or did, and in a fit of anger or confusion, they decided to leave. For example, if Hermione was working on a project alone without consulting them, they might have felt left out and left in a huff.
Perhaps in the fanfiction, Draco and Severus leave Harry because they are forced to by some external dark force. This could lead to Harry having to face new challenges alone, while Draco and Severus struggle against this force in their own way. They could end up being spies or trying to find a way to defeat the dark power from afar.
It could make Harry feel abandoned at first. He might have relied on them in some way, either for information or for support, and their sudden departure would leave a void. For example, if Severus was providing some crucial knowledge about magic or the dark arts, Harry would now have to find another source.
In fanfiction, one reason could be his desire for a fresh start away from the memories at Hogwarts. Maybe he feels he has done all he can there and wants to explore new magical knowledge elsewhere.
One reason could be to seek a new life away from the political drama in Konoha. Naruto has always been in the spotlight and has faced many political challenges. Hinata, being by his side, might support his decision to leave for a quieter place. Maybe they want to raise their family in a more peaceful environment, away from the constant threat of enemies targeting Konoha.
One possible reason could be that she feels she has outgrown the guild. Maybe she wants to explore new magic on her own and believes the Fairy Tail environment has become too comfortable and is hindering her growth.
The impact would be huge. Harry has always had this complex relationship with Draco and Severus. With them leaving, he might feel a sense of loss but also a new determination. For Draco and Severus, leaving Harry could be the start of a new chapter in their lives. They could end up in different parts of the wizarding world, encountering new people and situations. This could bring about a change in their personalities, and when they eventually cross paths with Harry again, they might be completely different characters.
One possible reason could be that in the original 'Harry Potter' lore, Draco and Harry are from rival houses and have a history of animosity. So in fanfiction, Draco might reject Harry based on this pre - existing enmity. For example, Draco's pure - blood supremacist views might clash with Harry's more inclusive attitude.
One reason could be to explore the wizarding world outside of England. There are many magical places they might be interested in, like the wizarding communities in France or America. They could be on a mission to learn more about different magical cultures and bring back new knowledge to England.