The other day I was browsing through one of the many Facebook groups I have joined, and I stumbled upon some author talking about how he or she managed to include symbolism in their stories. They found the common theme by the end of their first or second book, and they worked it into the entire story. Unfortunately, that could actually be a mistake!
Symbolism and themes definitely deserve a place in novels and fiction, but that place is somewhere in the background. If the theme is an overarching one that is the focus of the book, it can actually make your story a bit predictable.
One author talked about how he picked up a book series where the theme was cleansing, using water as the image. By the time he finished reading the first book, his mind was so absorbed with trying to predict how the series would use water again that he failed to enjoy the story.
How is this bad? Well, considering that a large percentage of your readers will be fellow authors, what you're doing is taking them out of the position of reader and transforming them into an observer--or, even worse, a critic.
If you follow a predictable pattern of imagery, themes, and symbolism, your readers will start looking for ways that you will include it into your work. They'll start critiquing the concept of your story, rather than enjoying the story itself. When that happens, you've nearly lost your reader, and it's very hard to get them back!
Themes, images, and symbolism definitely have a place in your book, but it shouldn't be at the forefront of your reader's minds. The symbolism should be subtle, hinted at instead of thrown in people's faces. If your story revolves entirely around symbolism, it will be almost too obvious. You want it to be in the background of their mind. You want them to say, "Hang on! That book/series was all about (X concept)", but you want them to say it AFTER they've finished reading the book.
Don't shove your imagery or symbolism down your readers' throats or make it too "in your face". Instead, assume that they're smart readers who will be able to winkle out the theme for themselves, and they'll appreciate the fact that you left it subtly in the background!