Having a personal project, or even something I'm just studying on the side, gives me a feeling of freedom and control over my life which I increasingly feel less of in other areas, with marriage, a mortgage, children, and especially a job with projects that I hate. Not that those things aren't all fine (well maybe not the bad job), but not being beholden to anyone else, having the pleasure to mess up or explore dead ends without worrying about the cost, are very nice in the face of all of life's other responsibilities. I think this goes back to Feynman's spirit of play, but there is more to it than just "having fun" in my opinion, at least it's a lot more complex than that for me personally.