A counterpoint to this is that many people -- myself included -- see that we spend so much of our waking hours at a job (about 40% assuming a 40-hour week) that we want to spend that 40% doing something that makes us happy.
So it's not that work defines us or drives our satisfaction, but that work makes up a large part of our lifespan, and it seems wasteful to spend it doing something just to facilitate the happiness of the other 60%.
I don't want to strive to be happy just 60% of the time -- I want to strive to be happy 100% of the time.