There could be several reasons. He might have been ostracized by his community, feeling all alone. Or perhaps he had made some terrible mistakes that he thought were unforgivable. Another possibility is that he was under a great deal of pressure, such as being expected to fulfill some impossible task and seeing no way out.
The guard's role is that of a savior. By stopping Merlin from committing suicide, the guard is preventing a tragic end. This shows the guard's sense of responsibility and his value for Merlin's life.
One possible reason could be the extreme pressure he faces. Merlin has to constantly hide his magic in a world where it's feared and persecuted. This secrecy and the fear of being discovered might have built up to a point where he feels there's no way out. Also, his loyalty to Arthur might play a part. If he feels he has failed Arthur in some way, like not being able to protect him fully from threats, it could lead to such despair.
It could also be due to some past trauma. Perhaps Merlin has been through an experience where Arthur was involved in a way that now makes him flinch. Like, Arthur might have accidentally hurt Merlin during a battle training in the past, and now Merlin flinches whenever Arthur makes a sudden move.
Sakura could be feeling inadequate compared to her peers. In the ninja world, there is always competition. If she constantly feels that she is falling behind in terms of strength or skills, it can lead to a deep sense of self - doubt and worthlessness. Also, she might have witnessed or been involved in some tragic events that she blames herself for, which could be another factor driving her towards suicidal thoughts.
One possible reason could be a magical curse. In many Merlin fanfictions, magic is often used as a plot device. A dark sorcerer might cast a spell on Merlin to make him forget certain things, perhaps to prevent him from using his powerful magic to oppose them. Another reason could be a self - imposed forgetting due to trauma. Merlin has witnessed and endured a great deal of pain and suffering. To protect his own mental state, his mind might subconsciously block out certain memories.
One reason could be to protect the person he marries. If she's a commoner, for example, the noble society in Camelot might not accept their relationship. So, they keep it a secret. Another reason could be that the person he marries has some sort of power or identity that could put them in danger if it was known. For instance, if she was a sorceress, in Camelot where magic is feared, they'd have to marry secretly.
It could also be due to the lack of proper rest. Merlin is usually depicted as a very busy character in fanfiction. He has duties in the castle, like serving Arthur, and on top of that, he has to use his magic to help those in need secretly. With so much to do, he might not have enough time to rest, leading to exhaustion.
Perhaps he has faced repeated failures in protecting his loved ones. If he constantly blames himself for not being strong enough to keep his friends safe, it could lead to self - loathing and suicidal tendencies. For example, if Sasuke leaves again and Naruto feels it's his fault, it might push him to the edge.
One possible secret could be that Merlin is a powerful sorcerer from an ancient and powerful magical lineage. Maybe his family has a long - standing connection to Camelot that has been kept hidden.
It could be that Gwen unknowingly has some sort of magical influence over Arthur. For example, if there is some ancient magic tied to her lineage that she's not aware of, and it's causing Arthur to focus only on her and forget about Merlin. Another aspect could be that Arthur's love for Gwen is so overpowering that it's subconsciously pushing out other important relationships, like the one with Merlin.