The tension in all stories is achieved by the adversity the characters must overcome. This is what keeps the reader's attention and keeps them reading. This tension delivers messages about facing challenges, overcoming difficulties and differences, discovering love, and how to become an ethical and understanding person. The plot will also contain a message about how the characters reach a final resolution.
Can a book have other messages? Messages that the reader must intuit? I think so, and in part, those messages might be what attracts the reader. What is amazing is that different readers can comprehend these messages differently.
Stories tell readers so much about humans, their characteristics, faults, and virtues. We are all different but all alike. A story can also reveal why individuals act the way they do. So reading might teach the reader understanding and how to deal with certain situations and people.
I think all of my books have these messages. The funny thing is that I probably didn't plan or recognize all the messages, not even as the author. In the Black Angel series, it is about the heroine finding herself after her mind has been destroyed. In the Homeworld series, it is about finding a home. In the Aegis series, it is about belonging and acceptance as the person you are. In Constantine's Legacy, it is about discovering a world-changing lie that cannot be stopped.
For other author viewpoints on this topic, visit the following posts: