For writers, theme boils down to why write this book. You could come up with an infinite number of scenarios, characters, worlds to create a piece of fiction. So, why are you writing what you are? What’s the point?
Readers may come to your book for many reasons. The theme, then, for them isn’t necessarily why they read your book, but what they take from it. What will they remember your book being about? What’s the point?
In that sense, coming up with a story shouldn’t be too difficult (though I say that knowing oftentimes how difficult it really is). We, as writers, just have to change our perspective. We need to look at the purpose of the specific story we want to tell. To analyze why we feel compelled to tell this story and what we hope to get out of it ourselves.
For example, Mean Girls. A hilarious take on the dramas of high school. Perhaps a bit extreme in the scenario, but that feels like the entire point (though I can’t say for sure, having not written it myself). If I were creating it, my ‘why’ would be something about the humor, about how we mature through drama, and about how important it is to be nice to other people.
As a viewer, I leave that movie having laughed my butt off. But, I also see those characters (though extreme) acting horribly to one another and know that I’d never want to do the same to others. That may be a subconscious change, but it’s still something I feel when thinking about the movie.
So, consider why you want to write the story you do. What’s in it for you? How will the story change your life? Or, get you to see the world a little differently? If you don’t have a good reason for writing it, maybe you should move on to something you feel closer to.