>Do you work exactly 40 hours? As in, start and 9am, leave at 5pm and never arrive early or leave late? Never a week of 41 hours? Because if I worked at a job I absolutely loved - as you describe - I would probably choose to work 50 or 60 hours. I think that the assumption here is that if you work 40 hours, you're probably doing it because that's what is expected of you and you're not willing to put in more.
Either that or you've put in anywhere between 50 and 80 hours in the past, burned out, and learned your lesson. Sometimes, I have to force myself to stop and go home after eight hours. In the moment, it feels horrible, as if I'm "quitting" or "giving up". I have to remind myself that I'm not quitting, but instead am forcing myself into a sustainable pace.
The marathoners learned this ages ago. If you allow yourself to take a fast pace right away, you'll burn out towards the end and won't have enough energy to finish. Debugging a subtle bug or implementing major new functionality is like a marathon. Even after you've accomplished the main effort, there are lots of little things that need to be done in order to go from "done" to "usable". Forcing a 40 hour work week ensures that you save some of your energy for the polishing and don't flame out after building the skeleton of the solution.