Jehovah's people are not happy people
During the 23 years since I broke from the JWs, I have only known one person who was diagnosed as suffering from "Chronic Fatigue Syndrome" (CFS).
This contrasts starkly with my final few years as a JW, where it was calculated that five percent of the publishers in our circuit were afflicted with CFS. Conspicuous amongst these were pioneers, elders (particularly those who were both elders and pioneers), and many others who were looked up to as outstanding in "their service to Jehovah". In other words, CFS tended to be a disease that particularly punished the more conscientious JWs.
For those less familiar with Chronic Fatigue Syndrome, one of its symptoms is a fearful, soul-destroying feeling of extreme depression. If you are unfortunate enough to succumb to this illness, one thing you aren't is happy! Furthermore, the standard JW cure-all for everything; i.e. more Field Service, more study of their propaganda, more meeting attendance etc. etc. etc. only further exacerbates the problem.