People often seem to think that I'm a workaholic and overly serious. I consider hockey to be my fun away from work and even then, I'm constantly reading up on techniques and working on ways to improve my skills. My teammates think I'm a little nuts. This season, I've decided that my focus will be skating, so I'm trying hit the rink twice a week for one hour skating sessions. GTD and lifehacking also can seem that way. People spend all this time to develop their productivity systems, boost their efficiency, etc and it's as if they're giving themselves more work (well, except when you have too much fun developing your system :) )
The best justification I can come up with, for the perhaps non-mainstream way I live my life, is that I believe in quality over quantity. I like to work 90% of the time to get that 10% of euphoric joy when I truly understand a physics concept, when I cap off a research project by writing a beautiful paper or give a lucid talk, when I rip a slapshot off.
Similarly, David Allen has said that he developed his GTD system so that he would have time to do what he really loves like bonsai gardening.
People often say that their greatest joy in life is their children and maybe what they really mean is that yes, it takes a ton of work to raise a child, but the joy they get from doing it is off the charts.
As a footnote, I should add that I personally find it aggravating to be bad at something so I also have a negative motivation for working hard! Both physics and hockey are difficult to learn and it's just no fun to do either if you haven't mastered the fundamentals. Of course, once you master the fundamentals of one stage, you move on to the next stage and sit around being frustrated with feeling stupid, work hard to learn enough to get past that stage and so on.