John Steinbeck wrote a famous letter about writing. Here's an excerpt and a link to the whole letter.
If there is a magic in story writing, and I am convinced there is, no one has ever been able to reduce it to a recipe that can be passed from one person to another. The formula seems to lie solely in the aching urge of the writer to convey something he feels important to the reader. If the writer has that urge, he may sometimes, but by no means always, find the way to do it. You must perceive the excellence that makes a good story good or the errors that makes a bad story. For a bad story is only an ineffective story.