For me, it's simple. If I didn't need to earn a paycheck I'd be tinkering day in, day out, chasing anything and everything that piqued my interest even a little. That's what I did when I was young with no responsibilities and no worries about the future, and it's my natural state.

That isn't reality, however, and so most of that energy is consumed by my day job, and it feels wasteful to put what little remains into projects that have little chance of any practical return. Any time I start settling into work taken up out of pure personal interest, the "responsible adult" part of my personality starts stratching at the back of my mind and pushing me to go do something more productive.

Such is life.