I wonder how much you get paid? I could see myself working just for fun, if I were certain that my salary will take care of me for the rest of my life. An example would be being an university professor with tenure, maybe? It would basically be advanced fuck-you money - you don't have the millions in the bank, but you have a guaranteed substantial monthly paycheck for the rest of your life. In corollary, whatever you do, you just do for fun. So you help out your company just for fun, and because you can.

Otherwise, much as I would like to donate my time to my employer for fun, I just can not afford it.