For me, it has to do with the sizzling energy that I feel while painting. There is something that needs to be said, or better, some wordless insight that demands expression.
And then, suddenly, the energy is gone. It has leaped like a spark from me to the canvas.
Of course, there are occasions when this intuitive sense of completion isn't accurate, when a work could use more work to express its message fully. That's when it becomes work instead of absorbing play.