Whatever the excuse, you are no longer excited to write the story.
You no longer love your characters.
Torturing your characters no longer gives you the thrill it used to.
You don't particularly care what happens at the end.
Were every last one of your characters to perish in a sudden ninja-zombie apocalypse, you'd be okay with that.
You've tried for weeks, and you just can't figure out why the antagonist wants what they want--or why the hero would be against that.
If you were reading this book, you'd put it down. Forcing yourself to continue feels as exciting as studying for the bar exam. Writing one more page will cause your brain to melt.
Last November, I participated in NaNoWriMo. All month, I pushed myself to write 50,000 words. And I did it. I wrote Every. Last. Word. Then, a few months later, I forced myself to outline (yeah . . . backwards). Most of the original 50,000 went in the wrong direction. (I'm not a very good pantser.) So I threw most of it out and started writing it again. I followed the outline. It was mostly interesting, but I just ran out of steam. I'd figured out what the antagonist wanted during the outline stage, but it made less and less sense as I wrote. Finally, though I was pretty sure what came next, it was just too boring to think about.
That's why I have this folder:
The Trunk isn't the end. Maybe I'll pull it out in a few months, dust it off, suddenly have an epiphany, and decide to finish it. In the meantime, it's just another half-finished novel. And that's okay.
I get to work on New-Shiny now.
Have you ever abandoned a WIP halfway through? Was it hard? Have you ever finished writing a book and wished you'd stopped halfway through?