1. Genuine intellectual interest
2. A desire to improve particular skills
3. A vague sense, acquired by osmosis, that industriously working on side projects during one's free time is what a Real Engineer does
Disentangling these motives and identifying a clear primary drive behind each project clears up the "hydra" feeling wonderfully, as most of the heads simply disappear once you realize that you never had a strong reason to pursue them in the first place. (3) in particular is often merely the self-castigating whisper of the internalized "should" rather than a valid reason to embark on a long and open-ended project.