The piece is done. It looks good. Anyone else would think it was fine. But you are looking at it from the maker's perspective.
There's that "one little thing" that insists on frustrating your sense of perfection. It might be a hard line that could be made sharper or a curve that could be just a little sweeter. Maybe there's small flaw in the wood that was filled and that now seem more conspicuous under finish. Or something in the finish itself that could use another little nudge.
I have been there so many times that I can't even guess at the number. And, more often than not, that one more little nudge does not go well. It usually leads to a whole lot of extra work trying to save a piece you just came close to ruining.
The idea that we can improve on our work is what drives most of us. If only we could learn when to stop!