Why this is a mistake: Theme or intent is the emotion you want readers to feel when they are finished with whatever you have written. No matter how objective you are, even if you are writing a newspaper article, your emotions are going to come through in your writing. If you are not aware of this, then they come through subconsciously and you can end up evoking in your reader a theme you didn’t intend.
Often, the theme in the works of a new writer comes out of the blind spot of his character, which means it’s often a negative theme, which might not be what he really wants to convey.
The solution: Theme comes out most clearly in the resolution of the novel, which is the last scene. So if you don’t know what the theme is before you write the book, at the very least, check it out after you’ve finished the first draft. But, of course …
If you don’t have a handle on your theme before starting, you can end up contradicting yourself in your own writing and confusing your readers. Also, while you don’t have to have a positive theme, ask yourself whether people prefer feeling positive or negative.