We're taught, when we're young, to finish what we started. When adults preached this, it was presumably because everything that we were supposed to do started and ended with a simple set of values that never changed. When your room's untidy, Mum isn't going to change her mind half way through the day, and say that tidiness is no longer her #1 objective. And when you've started sweeping the yard, Dad was never going to change his mind and decide that, hey, leaves are cool, and why don't we just leave them there?
But things aren't quite so simple in our professional lives: things change, and things can change pretty damn fast.