When you can state the theme of a story, when you can separate it from the story itself, then you can be sure the story is not a very good one. The meaning of a story has to be embodied in it, has to be made concrete in it. A story is a way to say something that can’t be said any other way, and it takes every word in the story to say what the meaning is. You tell a story because a statement would be inadequate. When anybody asks what a story is about, the only proper thing is to tell him to read the story. The meaning of fiction is not abstract meaning but experienced meaning, and the purpose of making statements about the meaning of a story is only to help you to experience that meaning more fully.
From Flannery O’Connor, “Writing Short Stories” in Mystery and Manners: Occasional Prose (1969). (Thanks to The Literate Kitten for recommending that I read this wonderful collection of essays!)