Theme is life's purpose.
Every story has a message, something to take away from the journey. When done poorly, theme ~is~ the message, and it’s obvious the writer is trying to preach this message. I’m not saying the writer shouldn’t have the intention to communicate something important and something they believe in. Rather, their message shouldn’t be obvious to the reader, and he certainly shouldn’t feel like he’s being taught a lesson. Instead, theme, when done wisely, only appears at the end of the story, from the story’s deathbed, as we reflect on the journey we just went through.
Likening this to our own lives, each day, we are going about our lives trying to do our best. We experience setbacks and learn, slowly zig-zagging towards an end. The days incrementally add up into life sagas. And the sagas into a life. Only at the end of life, upon reflecting backward, does a pattern emerge. Suddenly, a theme appears. A life purpose.
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One hack to finding the stories message is to look at the hero’s last meanignful decision. Often this symbolizes the theme, and his two paths in life.