He might obliviate everyone to keep a secret. And hide fanfiction because he thinks it's not suitable for public viewing.
Harry could be thinking of obliviating everyone to create a fresh start. As for hiding fanfiction, it might be due to some fanfiction containing false information about the characters or events. He may feel responsible for the 'true' narrative and wants to prevent the spread of fanfiction that goes against what he believes is right. Also, he may not want the fanfiction to overshadow the original work in a negative way.
He might choose to obliviate himself because of guilt. For example, if he made a big mistake during a fight that led to the injury or death of an innocent, he couldn't live with the memory. So, obliviating himself would be a way to escape that emotional pain.
It could be due to the saturation of the market. There might be so much Harry fanfic already that it's hard for new works to stand out. Additionally, changes in the way people consume media, such as a shift towards more visual content like streaming shows, could also lead to a decrease in interest in reading Harry fanfic.
The 'Harry and Daphne Obliviate' fanfiction concept is really cool. When Obliviate is involved, it can explore themes of power and responsibility. Harry, having experienced the misuse of powerful magic, should be more cautious. But if he uses Obliviate on Daphne, it might be out of good intentions like protecting her from a dark secret. However, Daphne might feel violated when she realizes her memories have been tampered with, which can lead to conflict between them and drive the plot forward in many different ways.
Perhaps he is trying to hide his true self. He may have a side of him that he doesn't want others to discover, like some special abilities or a different origin. So, being cold to everyone helps him keep his secrets and avoid getting too close to people who might figure him out.
One common plotline is that Daphne witnesses something she shouldn't, and Harry has to use Obliviate to protect her. Another is that an enemy uses Obliviate on Daphne to make her forget her connection to Harry, causing a rift between them. And sometimes, Harry accidentally obliviates Daphne's memories related to their relationship, and he has to find a way to make her fall in love with him all over again.
His closest friends like Ron and Hermione would probably be the most affected. They are used to having Harry around and relying on him. His sudden ignoring would leave them confused and worried.
It depends on the context and the rules of the story. Sometimes it's possible for plot reasons, but not always.
One reason could be that he doesn't want his friends to worry. Harry often takes on a lot of responsibility and might see his sickness as a burden he doesn't want to place on others. Another reason could be related to his enemies. He might fear that if Voldemort or his followers find out he's sick, they'll see it as a weakness and target him more aggressively.
Maybe he wants to keep it safe from thieves. There could be dark wizards or even some of his schoolmates who might be tempted to steal it.
Sure, there are. A lot of fanfics play around with the idea of what would have occurred if Lockhart had actually succeeded in Obliviating Harry. Some fanfics take it in a more comical direction, like Harry forgetting all his magical abilities but still somehow getting into all sorts of crazy adventures at Hogwarts. Others might be more serious, exploring how the wizarding world would have reacted to a memory - wiped Harry.