I was browsing a website called: My Unfinished Novels. It's basically a blog where people can put their unfinished books and explain why they never finished it.
The funny thing is that as I read through them, the biggest common thread I saw was that people lost interest.
Now I'm not saying that you should finish every single book you ever start. If you're bored writing it, chances are that people will be bored reading it. I've had some ideas that I wrote a few thousand words on and realized it wasn't a good idea after all.
But the real, true thing about writing is simply this: You have to freaking finish.
Writing isn't easy. It's grueling. It's a marathon. A triathlon. You don't have to finish first, but you do have to finish.
You don't become a winner by giving up at the half-way point.
That is all.