It's a very simple reason I keep writing.
The characters I'm writing will present an interesting tidbit about themselves I didn't realize until halfway through the story I'm writing.
Granted, sometimes the secret they reveal stops me, because I can't keep writing them into a plot if they no longer fit it. But sometimes like what my two main characters I'm writing in my alien invasion plot just told me, it gives me a reason to add a chapter to flesh out that new development.
I might actually get this one done, unlike the four other novels languishing in the first draft stage.