My goto on this topic is K.M. Weiland. I used to worry about how to have that added depth of resonance in a story because I wanted it there and still do. Even a 30-min sitcom episode has a life lesson embedded within it each week (before binge-watching existed).
Does every form of entertainment need to have a message of some sort embedded within? No, but most of what doesn’t seems to be slapstick comedy or gratuitous violence in some form. It’s just not my thing. I used to consider it low-brow entertainment but then I met a very intelligent friend who enjoys a much wider array of humor than I do. No more judgement. I altered my point of view to it just being a matter of taste. I still don’t get any personal enjoyment from those forms that start and end with the experience and leave nothing to ponder later. Now, though, where I used to feel elevated by my taste, I now have this vague feeling that something is wrong with me because I’m not like my friend who can appreciate the wider range.
That doesn’t change the fact that I like what I like and want to create similar experiences for others who like that thing. In this case, that thing is something that resonates in the mind after the cover is closed on the book; something that made me highlight or make a note of something from the book to think about beyond the story. In one of her blog posts on theme, K. M. Weiland writes about how discovering the theme should be like finding buried treasure. I agree with that and think that is a huge part of why I enjoy that extra aspect of a story. When I discover theme in some writing, I know that I am most excited about how the characters and story have been put together to evoke it, not even necessarily the theme itself. I might not even agree with the message sometimes but marvel at how it is crafted in the story.
Stories can have a theme or moral and be too obvious. I’ve actually declined to finish Brent Weeks’ Lightbringer series because of this. I enjoyed the first four novels of the series but the reviews for the fifth and final have a common theme in them that he has gone off the story and becomes very preachy and religious in the last novel.
I suspect that if you read through some of K. M. Weiland’s blog posts on the topic of theme, you’ll discover that your writing may already have themes buried within it. If your characters have any sort of change arc at all, it is hard not to have the seeds of theme there. Otherwise, why did they need to change?
I suggest starting with this post below and also following some of the links within the post to her other articles, if you’re interested. The best part is you don’t have to necessarily have the intention of theme and still have it appear with good plot and well-designed characters.