In fanfiction, it could be that his friends didn't support him in a crucial situation. Let's say Harry had a big dream or a goal, and instead of encouraging him, his friends were dismissive or even made fun of him. That would surely lead to disappointment.
It might be that his friends in the fanfiction started spending a lot more time with other people and sort of left Harry out. They could have made new friends and started ignoring Harry, which would make him feel disappointed as he thought they were closer friends than that.
Maybe his friends in the fanfiction betrayed his trust. For example, they could have broken a promise to him, like not showing up when he really needed them. This would make Harry feel let down.
Perhaps in the fanfiction, his friends failed to recognize his efforts. Harry might have worked really hard on something, like preparing for a big event or helping them in some way, but his friends didn't seem to appreciate it at all. This lack of recognition could easily make him disappointed in them.
In some fanfiction, the author might create a plot where Harry has been betrayed before. So, he has trust issues even with his friends. For example, if he was tricked by someone who seemed like a friend in the past, it could make him more cautious and less trusting in new situations with his real friends.
One possible reason could be to protect them. In many fanfictions, Harry might believe that by distancing himself from his friends, he can keep them out of danger. For example, if there is a new dark threat targeting him specifically, he may think that the enemy will use his friends against him.
It could also be due to a choice Harry makes that his friends don't agree with. Suppose Harry decides to take on a very dangerous mission alone to protect his friends, but they see it as him not trusting them. This could lead to a rift and eventually him losing their friendship.
Maybe his friends betrayed his secrets. In many fanfictions, a friend might accidentally or deliberately let out something Harry had confided in them, like his fears about Voldemort or his feelings for a certain girl. This could shatter his trust.
Well, perhaps one interesting moment could be when they are in a dangerous situation in the fanfiction. Harry might use telepathy to quickly communicate a plan to his friends without the enemy noticing. For example, he could send a mental image of a secret passage they can use to escape.
Telepathy likely deepens their relationships. They can understand each other on a more profound level. For instance, they can immediately know if a friend is in distress without having to say a word.
Often in fanfiction, Harry and Blaise become friends because the author wants to explore new dynamics or create unexpected bonds. It could be that Blaise shows a side that Harry connects with on a deeper level, beyond the typical school rivalries.
At first, Harry would be in disbelief. He would think it was some sort of joke or misunderstanding. But as the reality sets in, he would become a bit of a loner. He'd still have his sense of right and wrong, so he might continue to fight against the dark forces on his own, all the while hoping that his friends would come to their senses and realize they made a huge mistake in betraying him.
He would probably try to figure out what went wrong. Harry is a curious and determined character. He might start to recall recent events to see if there were any signs of his friends' discontent. He could also try to reach out to them, either directly or through mutual acquaintances, to ask for an explanation.