Jealousy could be one reason. Maybe his friends are jealous of his achievements as a Pokémon trainer. Another reason could be that they are influenced by some external forces, like an evil organization that convinces them to betray Ash.
One common cause is misinformation. If false rumors are spread about Ash, his friends might believe them and turn against him. Also, sometimes in fanfiction, a friend might have a hidden agenda. For example, they might want to take Ash's Pokémon for their own gain and so betray him to achieve that. It could also be a matter of loyalty being tested in a difficult situation, and they fail the test when it comes to Ash.
One possible reason could be jealousy. Maybe his friends are jealous of Ash's success in his Pokemon journey. For example, if Ash wins a big tournament and gets all the fame, his friends might feel left out and betrayed. Another reason could be a misunderstanding. Ash might do something that his friends misinterpret. Say he makes a deal with an enemy in order to save his Pokemon, but his friends don't understand the situation and think he's turned on them.
Well, one reason could be jealousy. Maybe his friends envy his fame or special abilities. Another could be misinformation. If they are fed false information about Harry, they might turn against him. For example, if someone convinces them that Harry is secretly working for the enemy.
One possible reason could be jealousy. Maybe Ash has achieved more in his Pokemon journey than his friends, and they can't handle it. Another reason might be some misunderstanding. For example, an enemy could trick Ash's friends into thinking Ash has done something bad. Or perhaps there's some sort of power struggle. If there's a powerful item or a rare Pokemon involved, and Ash is the main contender, his friends might be tempted to betray him for their own gain.
In such fanfiction, it can be a complex situation. Maybe Ash's friends were misled or had their own hidden motives.
Betrayal could also stem from a misunderstanding. Maybe Ash or Cynthia accidentally offend someone without realizing it. For example, they might unknowingly step on a trainer's dreams or beliefs. And that trainer, feeling deeply hurt, decides to betray them. Or it could be that a Pokemon they both have a connection with has a different loyalty that they were not aware of, which leads to the Pokemon causing their betrayal.
One possible reason could be jealousy. Maybe his friends are jealous of Sonic's speed or his heroic status. Another reason might be a misunderstanding. For example, Sonic could be framed for something he didn't do, and his friends believe the false evidence. It could also be due to outside influence, like an enemy manipulating his friends' minds into thinking Sonic is a threat.
One common theme could be Ash's feelings of isolation and despair. After being betrayed, he would likely feel very alone and question his past relationships.
One reason could be jealousy. Maybe Naruto's success as a ninja makes his girlfriend jealous and she betrays him out of spite. Another reason might be that she is forced to by an enemy. For example, if she is blackmailed into betraying Naruto to save her family.
Ash would probably be devastated. He would feel like his world has been turned upside down. At first, he might not believe it and think it's all a big mistake. As the reality sinks in, he could start to isolate himself from others, even from his Pokemon for a while. However, his strong will would eventually kick in. He would use this as motivation to become stronger, not just in his Pokemon battles but also in his ability to face challenges alone, without relying on his so - called friends.
Maybe in the fanfiction, his friends keep getting in the way of his goals. For example, if Ash is really focused on becoming a Pokémon master and his friends are constantly causing distractions or making him take detours from his training and battles.