Where does motivation come from?
The idea of the carrot and the stick suggest external forces: we chase the carrot--the external reward--and we flee the stick--external punishment. It seems to me that both of these forces of motivation are impoverished with respect to character growth.
If we look at the issue using the perspective of Maslow's hierarchy of needs, we might see the carrot and the stick as fulfilling lower-level needs--safety, security, pleasure.
What have we got left if not the carrot and the stick? Maslow would suggest we have our growth and our joy in the mastery of new things. (or is that a metaphorical carrot)?
Ach! I'm getting tangled in metaphor. The point I'm really aiming at is the value of being accountable to yourself rather than to someone else. Do the work because it will satisfy what you want to accomplish in your life. Do it for yourself. Do it because you have a sense of purpose that will be served by working on your project.