Ask HN: Why/how do you choose to work on side projects?

Ask HN: Why/how do you choose to work on side projects?
3 by _netlurker | 3 comments on Hacker News.
I guess this is just my poor perspective on the whole thing, but I keep seeing people working on not-your-typical-stuff kind of projects such as compilers, computer graphics, low-level stuff in general, and I keep wandering whether those people do this as part of their line of work or just have enough time after work to learn about those fascinating things on their own, and what the reason is behind it. This is also sort of dilemma for me because I can't find the best strategy as to how to approach self-learning. On one hand, I want to improve in my job, so I can do it better, but on the other hand I want to have a hobby or work on other skills to move on to another job. What is your reason for working on side projects and how do you select what to improve on next?