When one writes fiction, the readership tends to think one of two things. Either, “This is all true, s/he just put different names/places/times for all the people/places/things in involved,” or “This is all completely made up, there is nothing of the author’s personal experience in this piece.” The latter opinion is more heavily adopted the farther from normal reality the writing is.
Both are wrong. It’s called fiction because it’s made up, but a person can’t make up something from nothing. There is always a path that has lead the author to the made up something. So even in the most outlandish or silliest of stories, there is a deep psychological piece of the writer present.
That piece might be an escape. “Oh, I wish the world were like this!” It might be an exploration of why awful things happen, “Why did that serial killer kill all those people?” Or, it might be an exploration of why beautiful things happen, “How did that man become so happy?” I’ve written and enjoyed the writing, of all three of these scenarios.
As a reader, though, you’ll find a theme in an author’s work. The reason for that is the deep psychological piece of the writer that is present in the work. Consequently, the author’s fears, desires, wishes, dreams, frustrations, and just boring reality creeps into his or her work. It is unavoidable. However, to mistake the writing for the writer is to give both a deep disservice.