I generally hate the belief, that people are unmotivated because the work "was too easy/not challenging enough," since the ease of something probably bears little relationship to its motivation. Yes, some people like challenge, myself included, but motivation often stems from seeing the goal as having worth, either to one's growth, or to some larger goal. Challenge can be good, provided one believes there is a real solution, or that one's efforts would be rewarded, but motivation always matters more.
Motivated people don't see challenges, they see opportunities, but you have to get to the point where the task is rewarding. Even then, real people have broader needs than simply goal orientation. Typical motivations:
- Money (not my favorite, but a favorite cliche)
- Social obligation (make other people happy)
- New technology (some thrive on novelty)
- Work with friends
- Advance one's skills
Although this might seem common, it is probably forgotten, particularly in the world of men that coders inhabit, that feelings matter.