Why do you work? Perhaps money will come up as the main answer, but if you think about it, work is not all about money. People have left high paying jobs for various reasons (myself included). At the end of the day, I think that reasons for working are so complicated that it can't simply boil down to money (although that does play a large role).
That image above is a wonderful flowchart which you can check out if you are presented with the opportunity of working for free. Pro bono. No pay. Zilch. Zero.
Click on the image for details.