sometimes work becomes work. I really enjoy providing control systems for hydro electric power plants, but when people change their minds and I have to re-do the same work, or if some equipment provided by another firm doesn't work properly and it eats up a bunch of my time it is a drag. Or I get burnt out after working 12 hour days for four months so that we don't get sued if the power plant isn't generating on time, and then I might have loads of work to do after to clean up the documentation and other parts of the contract so we can get paid the full amount, but man I just don't feel like working anymore, whereas if I'm doing a moderate amount every day with lots of time for exercise and socializing I really enjoy the same work.
edit: I should add that I work either on site at power plants or remotely from home