It takes a lot of practice to get to the place where you can sense what to add and what to leave out. Think of it as stagecraft. Even in nonfiction your story needs a setting and your characters need a history, but you only have so much room and so much time, so you have to pick and choose. What's essential and what's not? The hardest part for writers is cutting something you know is superfluous but you really, really love. That's when you become the editor. But if you love it and just can't let it go, tell the editor to go to hell. It's staying! Because you're the boss. LOL.