Merlin is a very interesting character. His magical powers and his role as a servant with a big secret in the original story make him a prime target for fanfiction. People like to write about him because they can explore his untapped potential in their stories.
There are several reasons. First, Merlin's character has a lot of room for development. In the original, there are many aspects of his life and powers that are only hinted at. Fanfiction allows writers to fully explore these. Second, his relationships are very interesting. His friendship with Arthur is iconic, and fanfiction can delve deeper into their bond, adding new twists and turns. Also, his connection to magic in a world that is often afraid of it gives a lot of scope for creative storytelling in fanfiction.
It could be for comedic effect to lighten the mood and add some fun to the story.
Because both are related to magic. Harry Potter is all about magic in Hogwarts and the wizarding world, and Merlin is a famous magical figure. So, the fanfiction about Merlin might offer a new perspective on magic that Harry would find fascinating.
Because he loves magic. Merlin's world is full of magic too, so he might be curious to see how other magical stories are told.
In Merlin fanfiction, Merlin calls Arthur sire as a sign of respect. In the Arthurian legend and related fictional settings, sire is a term used to address a nobleman or a person of higher status. Merlin, being Arthur's servant and advisor in many versions, uses this term to show his loyalty and deference to Arthur's position as king or prince.
It could be due to jealousy. If Merlin gains too much power or popularity in Camelot, Arthur might feel threatened and in a fit of rage, stab him.
In some fanfics, Arthur might be mean to Merlin because of jealousy. Merlin has magic, a power that Arthur doesn't fully understand and might envy on some level. Also, Arthur's position as king or prince makes him feel like he has to be tough, and Merlin, being his servant, is an easy target for his pent - up frustrations.
One reason could be that other characters are more popular. For example, if there are more 'flashy' or relatable characters in the source material, they might draw more attention in gen fanfiction.
Well, in many fanfictions, Arthur might be overprotective of Merlin because of their deep friendship. They've been through a lot together, facing numerous dangers and challenges. Arthur has come to rely on Merlin in many ways, and he doesn't want to lose him. So, he becomes overprotective to ensure Merlin's safety.
In fanfiction, Arthur might be rough with Merlin because of a prophecy or a pre - ordained fate. He might think that by being tough on Merlin, he can prevent some bad outcome that he believes is related to Merlin. For example, if there's a prophecy that Merlin will bring great change to Camelot, Arthur might be rough in an attempt to keep things as they are, out of fear of the unknown.
Maybe it's a name from Merlin's homeland or has some special meaning in the lore of the fanfiction world.