It might start with small incidents. For example, Ron being late to meet Harry a few times without a proper explanation. This can plant the seeds of doubt in Harry's mind.
Another reason could be personal insecurities on Harry's part. Harry has been through a lot and might be over - analyzing Ron's behavior. He could be projecting his own fears onto Ron. For example, if Harry is worried about being betrayed again like he was by some people in the past, he might start to doubt Ron even without solid evidence.
Open communication is key. Harry and Ron need to sit down and have an honest conversation about their feelings. Harry can tell Ron why he has started to lose trust, and Ron can clarify any misunderstandings.
Perhaps there were some secrets that Ron was keeping from Harry. In the wizarding world, secrets can be a big deal. If Ron had a secret related to his family or some other important matter and Harry found out about it in a way that made it seem like Ron was hiding something bad, it could lead to Harry not trusting him. Also, Ron's friendship with other characters might make Harry jealous or suspicious. If Ron was getting closer to someone Harry didn't trust, it could spill over and make Harry question Ron's loyalty to him.
The dynamic becomes very strained. Harry's lack of trust in Ron means that their usual banter and easy camaraderie is gone. Hermione will be stressed because she values both of their friendships. She may end up spending more time with Harry to try to help him through his doubts, which could make Ron jealous. Ron, on the other hand, might become defensive and start acting out, which only makes Harry trust him less. This could lead to a cycle of misunderstandings and a breakdown of their friendship triangle.
One possible reason could be miscommunication. Maybe Ron said something that Harry misinterpreted, leading to the distrust. For example, if Ron made a joke about Harry's abilities in a time of stress, Harry might take it the wrong way.
Well, in some fanfictions, Harry might not trust Dumbledore because Dumbledore often keeps secrets. For example, he doesn't fully disclose the prophecy details at once. Harry could feel like he's being manipulated or not given all the information he needs to face the challenges. So, that lack of transparency can lead to Harry's distrust.
In some fanfictions, the author might want to create a new plot twist. Maybe they introduce a misunderstanding, like Hermione accidentally getting involved in something that looks suspicious to Harry. For example, if she's caught in a secret meeting that she can't immediately explain, Harry could start to lose trust.
Maybe in the fanfiction, Harry has had bad experiences with adults. For example, some adults in the story might have betrayed his trust in the past, so he becomes wary of all adults.
Maybe he has read some really bad Order fanfictions before that were full of inaccuracies or out - of - character portrayals, which made him lose trust in this type of fanfiction.