One of many possible reasons is, to me, the fact that you need to enjoy every step of the task/job to keep that motivation high.
In other words, motivation is happiness.
And that's what Dan Ariely explains brilliantly in the video below.
There was one sentence I wrote down:
Ignoring the performance of people is almost as bad as shredding their efforts in front of their eyes.
And he's damn right...
Enjoy the video.