I recently heard a great interview with Barbara Kingsolver in which she compared the difference between writing fiction and writing nonfiction to the difference between gardening in the desert and gardening where it's lush.
When you garden in the desert, you point to a spot on the ground, bring in soil, fertilizer, and seeds, and you water, water, water. Basically, you supply everything yourself, creating something from nothing. Like you do when you write fiction.
When you garden where it's lush, you point to a spot on the ground and then get rid of everything you don't want or need -- vines, leaves, and weeds, weeds, weeds. Then you plant your garden and continue to do battle with those pesky weed intruders, which are always competing to share space and nutrients with your flowers and vegetables. This is like writing nonfiction -- I'll narrow it to narrative nonfiction, though it could apply to all nonfiction if we used different vocabulary -- when you look at everything that happened in the universe of the story you want to tell, and then you get rid of each thing that doesn't support that story. You eliminate things that weaken or don't serve your narrative arc, your character development, and your theme. Even if something's interesting and it really did happen (honest it did), if it doesn't support or add to your story, you pull it out. Because ultimately it's a weed, even if it's a really nice one.