Perhaps Bucky believes he is protecting Steve in a very twisted way. He might think that Steve is in danger from other forces that he can't explain to Steve directly. So, instead of talking it out, he kidnaps Steve to keep him safe from what he perceives as a greater threat.
One possible motive could be that Bucky is being controlled against his will. In the Marvel universe, he has been brainwashed before. Maybe the people who once controlled him have regained their hold and made him target Steve for some strategic reason, like using Steve as a hostage to gain power or information.
In some fanfics, a motive could be a deep - seated jealousy. Suppose Steve gets all the glory and recognition while Bucky feels left in the shadows. This could build up resentment over time and lead to a situation where Bucky snaps and kills Steve. However, this is a very different take from their normal relationship in the official stories and really plays with the emotions and expectations of the readers.
It could also be a result of some external influence, like a dark magic spell that has clouded Ron's judgment. This spell could be making him act against his normal character and do something as extreme as kidnapping Hermione. However, in the end, he would likely break free from this influence and realize the error of his ways.
One possible motive could be to protect Sasuke. Itachi might have seen a great danger approaching Sasuke that he could only avoid by taking him away forcefully. Maybe there was a powerful enemy targeting Sasuke, and Itachi thought that by kidnapping him, he could keep him safe in a hidden location and train him to become stronger.
One possible motive could be misinformation. Maybe Bucky was misled to believe the OC was a threat to someone he cares about. So, he kidnapped the OC to prevent them from causing harm.
Maybe Bucky kidnaps Steve because he believes it's the only way to protect him. There could be a new and powerful threat that Bucky has discovered, and he thinks that by taking Steve away from the public eye and the Avengers, he can keep Steve safe. But Steve doesn't understand Bucky's motives at first and there's a lot of tension between them until Bucky can explain.
It could also be out of love and a sense of family. Sirius sees Harry as the only family he has left. He might fear that Harry is growing up too fast in the wizarding world, surrounded by danger and politics. By kidnapping him, he hopes to give Harry a normal childhood away from all the chaos, even if it's in an unorthodox way.
One possible motive could be Anakin's fall to the dark side. The dark side clouds his judgment and makes him see Obi - Wan as an obstacle to his power. He might believe that by eliminating Obi - Wan, he can fully embrace the dark side and gain more control. Another motive could be jealousy. If Anakin feels that Obi - Wan is always favored or is holding him back in some way, this could fuel his anger and lead to him wanting to kill Obi - Wan. Also, the influence of Palpatine might play a huge role. Palpatine could have manipulated Anakin into believing that Obi - Wan is a threat to the new order he wants to create.
It could also be a personal vendetta. Maybe she had some sort of conflict with someone in the past, and that person decided to kidnap her as revenge. For example, if she had testified against someone in a court case, the person might want to get back at her.
Sasuke might want to get some information from Naruko. Maybe he suspects that she knows something about a threat to the Uchiha clan that he is trying to uncover.
One possible motive could be for a prank. People might want to give their friends or family a big scare and then have a good laugh about it later. For example, like the stories where friends plan a fake abduction to surprise the victim with a party or some other fun event.