Because if I didn't, I'd go insane during the writing process. I have to know what's coming next -- it's the whole obsessive-compulsive personality trait, I think. Afterburn is making me crazy with how slowly the plot is unfolding in my head. I do believe school stress is getting in the way. I hate to see what happens if I actually take on the second job I'm looking for . . . but one must do what one must do, right?
I was thinking about plotting today, while I was teaching (well, covering) a class for a colleague who was in a meeting. It was an English class and the kids were researching, so I didn't have much trouble with it. But I know I function much better when I've worked out a lesson plan ahead of time -- everything from assessment to what the kids need to know and do to what standards I'm addressing and how I'm going to approach the lesson.
Writing a book? Same principle. I need to know where I'm going, where the turning points in my plot are. Does the book often turn out differently than my plan? All the time, but I think that's normal. I still end up in the same place, just with a slightly different route.
I can't imagine writing without a map, though.
What about you? Plotting or no plotting? And why?