You don't discuss it with anyone, so all your writing energy goes into the story, into moving it forward and making it better.
It's just for you. Maybe someday others will get to read it, but in the beginning it's just the two of you.
There's immense freedom in that, in knowing that nobody else's expectations will sway it. Nobody's criticism matters; nobody's approval is necessary.
Nobody else is awaiting it, so even if you stop dead in the middle of a sentence and never touch it again, it's OK. If you write 5000 words a day on it, it's OK. If you do just one draft, it's OK. If you do 71 drafts, it's OK.
Everything's OK, which is the really marvelous part. You are not worried about selling it. You are not worried about anything, really.
I have had secret projects, and they were fun. Maybe they're not for everyone; some people like to talk through plotlines and characters as they write. But this post is for the ones who hug a work-in-progress close to the vest, maybe not even admitting that it exists. For a while, anyway.
Have you ever had a secret project?