Why do people do what they do? That's what motivation is about. David McClelland, the guy who really made this field work, started thinking about that when a child wondering why people behaved differently on Sunday in church than they did the rest of the week.
At the risk of oversimplifying, there are three large categories of motivation:
"Want" is shorthand for the emotional drives, where you find enjoyment, energy, or frustration. Mostly what I'm talking about here. "Should" is shorthand for values, conscious beliefs, things you have decided are important. "Must" is shorthand for things other people force you to do, or that you get externally rewarded to do.
Motives and values are both internal motives, to different degrees--they come from you, and it is internally satisfying. "Must" things are external to you.
Money is an "extrinsic reward." Emotional satisfaction is an "intrinsic reward." Despite what many people think, there is no correlation between the two! There are things you do that are satisfying and important, things that are just fun or just important, and things which are neither.
The odd thing is that some studies have shown that extrinsic rewards can squelch internal motivation. In other words, in some cases money decreases motivation. Why? Well, it seems to be because it takes away the fun--now it's a job, and it's not something you are choosing for yourself. One simple study had people doing little puzzles (e.g., finding "Ninas" in Al Hershfeld drawings) supposedly while they waited for the experimenter. When the experimenter came in and said they were actually doing the study and would be paid for their performance, people refused the money.
This is obviously particularly germane to writing. I've contended here that published writers typically write to be read. Various people who have posted here about fan fiction have said either they are happy to write for a small, knowledgeable group or they write for their own pleasure. (Some have said they do want to publish conventionally but for whatever reason have not tried or have no outlet.)
It's my belief that enjoyment or emotional engagement should ALWAYS be the fundamental reason you write. Not necessarily the only one. But even for those who are writing for money or even making a living at it, the slow, small, and irregular nature of pay doesn't make it a very effective extrinsic motive either! Especially since extrinsic motivators can inhibit the fun, and when they are removed, the productivity disappears as well. In other words, if you are relying on something outside you to motivate you, if the situation changes, so does your motivation. Not a good formula for extended effort.
Hunter S. Thompson once wrote that he didn't usually enjoy writing until he wrote Fear and Loathing in Las Vegas, because when "you have to do it," it gets old. Same principle. As he put it: "old whores don't do much giggling."
Besides, anyone smart enough to make money at writing is probably smart enough to make more money at something else! Then you can have fun with your writing again.