One reason could be related to Arthur's relationships. If Arthur has a love interest or close friendships that Merlin wishes he could have with Arthur too. Like, if Arthur is spending a lot of time with a particular noble or a lady, Merlin might be jealous because he wants that same level of closeness with Arthur.
It really depends on the situation. However, Merlin could also be very jealous in a romantic context. He has a deep connection with Arthur and if Arthur were to show interest in someone else, Merlin might feel jealous. Merlin is often more in touch with his emotions, so his jealousy could be more intense and internalized compared to Arthur's more outward expressions of jealousy.
One common theme is the jealousy over their respective friendships. For example, Arthur might be jealous if Merlin spends too much time with other knights. Another theme could be romantic jealousy. If there are love interests involved for either of them, it can lead to some intense jealous moments. Also, jealousy over power or status can play a part. If Merlin's magic gives him an edge in certain situations, Arthur might feel a twinge of jealousy, and vice versa.
One common theme is the struggle with unspoken love. Arthur's jealousy often arises from his love for Merlin that he can't or won't admit. It's a way of showing his possessiveness in a romantic sense without being fully aware of it himself.
The banter between Merlin and Arthur in fanfiction is really interesting. It can range from light - hearted jabs at each other's habits. Merlin might poke fun at Arthur's love for jousting and how he always gets a bit too competitive. Arthur could retort by mentioning Merlin's clumsy attempts at serving him. Their banter also reflects the underlying respect they have for each other despite their different social standings. It's like a friendly sparring that keeps their relationship dynamic.
The 'Redemption' trope is also seen in some fanfictions. Maybe Arthur has made some mistakes in the past, and Merlin helps him to redeem himself. Or vice versa. It's a way to explore the characters' growth and development. And then there's the 'Battle Against Evil' trope where they fight side by side against dark forces, which really showcases their bond and loyalty.
I really like 'The Sword in the Stone' as a fanfiction - like take on Merlin and Arthur. It shows Merlin as this wise and mysterious figure guiding Arthur on his journey to become king. There are also some great interactions between the two characters that show the deep bond they share. You can find it in many libraries or online bookstores.
One popular 'jealous Arthur x Merlin fanfic' is 'The Unspoken Feelings'. In it, Arthur's jealousy over Merlin's interactions with other knights is subtly portrayed. It shows how his jealousy stems from his deep, unacknowledged feelings for Merlin.
Merlin would likely be seen as a bit of an enigma at Hogwarts. His magic is different from the norm, so he might be a bit of an outsider at first. But his kind heart and loyalty would soon win over his classmates. Arthur would be the brave and headstrong character, perhaps a bit like a Gryffindor version of his usual self. He'd be eager to learn about combat magic and would probably be a natural leader in group projects or Quidditch teams.
During the time of Merlin's captivity, they could have long conversations that they never had before. Arthur might open up about his insecurities regarding magic and his kingdom, and Merlin could share his true feelings about serving Arthur. This would strengthen their bond in a new and unexpected way.