Every good story has a theme in it.
Theme: Universal truths and concepts about life.
In stories, themes are lessons where the audience is expected to extract meaning for themself in an organic way.
This means you cant have some wise character telling your readers to accept the theme “Because I told you so.”
We all find it more rewarding to learn something for ourselves. By teaching through theme, we give only “What Happens” and allow readers to create them own beliefs form the information.
Too many bland stories feel the need to lecture on subjective principals. Telling your audience to feel a certain way makes them more resistant. If they learn it on their own, they will be more sympathetic.
Common themes might as simple as Good vs. Evil, Guilty until proven innocent, and there are two sides to every story.
Complex themes can be more powerful at times because they aren’t as cliche. there is nothing wrong with using Good vs. Evil, but it is used a lot.
How does this sound? All violent people should be publicly murdered in front of their families (I know, intense example, but it caught your attention right?)
Tying theme into your story will help to create characters that feel real, make the story more interesting, and hopefully leave readers feeling like they gained something by reading you novel.