There are many possibilities: * After a few months, the hardest parts of the problem have been solved. Since your skill level exceeds the challenge, boredom r

There are many possibilities: * After a few months, the hardest parts of the problem have been solved. Since your skill level exceeds the challenge, boredom results. See: http://en.wikipedia.org/wiki/Flow_(psychology) http://en.wikipedia.org/wiki/Flow_(psychology) * Perhaps you are not motivated by the actual task, but by the sense of mastery. Once you've mastered a challenge, you are satisfied because you've learned something about yourself: I can do that! The project still requires work to be done, but you're interested in pursuing some other new challenge that validates your sense of self-mastery. * If you're cycling, perhaps you have a touch of bipolar. * Perhaps "How can I stay enthusiastic?" is the wrong question. Maybe "How can I persevere when my enthusiasm inevitably wanes?" Expecting to always be enthusiastic is unrealistic.