Sometimes, it's a plot device to create tension. A scared heroine adds an element of mystery and drama. The author might use this to gradually develop the relationship between the two characters. As the story progresses, the hero can then work to gain the heroine's trust, making the romance more interesting.
One common situation is when the hero has a secret. If the heroine senses that the hero is hiding something, it can make her scared. For instance, if he's part of a dangerous undercover operation but she doesn't know it, his evasiveness can be frightening to her.
It could be because of her unique personality or charm that catches his attention and makes him unable to resist.
They offer a sense of innocence. It's like seeing love in its purest form, untainted by past experiences. For example, in many such novels, the characters are experiencing love for the very first time, and that makes their emotions more intense and relatable.
It could be that the hero is so focused on other goals, like achieving a great career or avenging a wrong, that he doesn't have time for the heroine. He might think that romance would be a distraction from what he really wants to do. For example, if he's on a mission to save his family's honor, he might see a relationship as something that could get in the way of his pursuit of justice.
Maybe she has had some bad experiences related to so - called heroes in real life, which makes her cynical about the idealized heroes in romance novels.
Redemption can be a theme. Maybe the hero has a past that makes him seem more threatening, but as the story unfolds, he redeems himself in the eyes of the heroine. Also, the theme of love conquering fear is quite typical. Despite her initial fear, the heroine gradually falls in love with the hero, showing that love can overcome her apprehensions.
Often, the hero being mean at first builds up to a big transformation and a deeper connection later. It could be a plot device to make their eventual love more meaningful.
These plots are popular as they play on the idea of 'opposites attract'. The hero, who is often seen as the 'bad guy' at first for kidnapping, turns out to have a soft side and falls in love with the heroine. It's a classic trope that has been used in many successful romance novels. It gives a lot of room for character development and emotional growth as the two characters get to know each other under such extreme circumstances.
Sometimes, it's a plot device to create conflict and tension. The mistreatment might lead to character growth and a more meaningful resolution in the story. Maybe the hero has misperceptions or misunderstandings that cause his behavior.
Often, it could be due to a lack of communication or understanding between the two. Maybe the hero is used to having things his way and doesn't realize how his actions affect the heroine.