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.
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.
One reason could be for the continuation of his dynasty. In the context of Camelot, having a son would mean there is an heir to the throne. It would ensure the stability and future of his kingdom. Without a son, there could be disputes and power struggles over who will succeed him.
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.
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.
Merlin might have used his knowledge of the castle's secret passages. He could lead Arthur through those passages, away from the assassins. Also, Merlin could have called upon some magical creatures he's befriended, like the dragon, to help him. The dragon could fly in and either scare the assassins or carry Arthur to safety.
Merlin could be heartbroken. He might retreat into himself and become more withdrawn. After all, he has a deep bond with Arthur, and this sudden break - up would be a huge shock.
Merlin would be very worried. He would probably stay by Arthur's side day and night, using his magic secretly to try and heal him. He might also search for rare herbs or magical items that could cure Arthur.
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.