I reread  Are Wireheads Happy?[1], which is about the difference between liking and wanting.

I've noticed that too much simple carbs (not terribly much-- three twenty ounce Cokes on consecutive days will do it) severely damages my desire to do much of anything, and the effects take approximately two low-simple carbs days to clear out.

This is obviously something physiological, even though it looks like an emotional problem. Failing to remember clue to avoid simple carbs (a dessert once a week might not be a problem) might be an emotional issue. Too much sugar used to lead to an internal voice saying "I don't care". This time, it didn't, but if I thought of something I might do, there was a clear feeling of "no reward there" and a sense that it was too much effort. I was capable of enjoying things, but not of anticipating that I would like them.

I'm wondering if anything is known about simple carbs, motivation, and/or serotonin/dopamine.

[1]Recommended here.