You have to get something out of it.
It's the best when the process alone makes you happy, but this isn't usually the case 100% of the time.
The next best thing is getting results. Either because you like what you created or other people like it.
Writing my 50 blogposts in 2017 wasn't always fun, but some of them got thousands of views.
I wonder though if there is a reluctance to adopt an approach similar to King's because people are wary (possibly subconsciously) of killing their enjoyment of the process: 'Don't turn your hobby into a job' is advice that you hear a lot.
Perhaps those who are prolific have their eyes more on the end result (a good book in 3 months) rather than the act of writing and that helps them deal with this as well as being more generally motivating to produce good work.