There is also the component of being sad and not knowing why, while you work on what you think is interesting and important. I remember reading how Feynman was

There is also the component of being sad and not knowing why, while you work on what you think is interesting and important. I remember reading how Feynman was unhappy with his work and then chose a seemingly useless but fun and interesting problem to solve - the physics of plate wobbling (If I remember correctly, the relation between the wobble rate and spin rate of a plate that has been tossed into the air). His colleagues were a bit confused as to why he would do this, but he had fun and his love for physics was rekindled. Some years later the mathematics he derived would be used when the first satellites where launched and wobbled as they spun, the wobbling not being desirable. Not bad for useless and fun work!