I was talking on the phone Saturday with a friend who called to wish me happy birthday on my 65th birthday. (See what I did there? :-)) This friend is a doctor, not an economist, but I think he would have been a first-rate economist. He's a fan of Econlog.
I told him that, contrary to my usual practice, I planned not to work on Saturday. That got us talking about work and enjoying work. I quoted an economist friend of mine, Tom Nagle, who is also a fan of Econlog, who said some years ago: "I know many people who love their job. I know of no one who loves his job on the margin." That made sense to me.
I then told him about quoting Tom Nagle's line to Dwight Lee once, who, quick as a flash, shot back, "Of course people don't love their jobs on the margin. If they did, they would be out of equilibrium."
In other words, if you love that marginal hour of work, you're getting two returns from it: (1) the income and the (2) intense pleasure. Then the utility from the income plus the utility from the intense pleasure is highly likely to exceed your opportunity cost. You're out of equilibrium. So you will work longer. QED.