If we find the cause or a partial cause of a syndrome, it will get a new label. That is why "AIDS" has given way for "HIV infection", "Scurvy" for "Vitamin C deficiency", etc. CFS, being a syndrome, may Or may not have a single cause. We simply do not know.
A (IMO) problem with such vague syndromes is that, in general, it pays of to claim to be part of a a larger group. It is more accepted to say your son has ADHD than to say he cannot concentrate well, or that one has CFS than that one is lethargic., does not feel like doing much, etc.
Net effect is that, even if there is a group of true patients with well-defined symptoms with a common cause, it is hard to separate them from "me too" persons (a group that may include persons that have a different unknown ailment with similar symptoms)
My brother and I both got CFS at the same time, with similar initial-diagnoses (tested positive for Mono), and we've been fighting off the effects for quite a few years now. For myself, I went from soccer-playing high-energy kid to going between bed and the couch for about a year and a half, in the span of a few days.
What changed after a year and a half was that I was getting ever-deeper in a depressive cycle, and managed to recognize it and escape - I realized it was literally a lethal path if I didn't fix something. Very good motivation, that. From then, I basically spent 5 years wasting a little more energy in everything I did, and very very slowly got some stamina back - it took me about 5 years from the start of it all to get to the point where I could go back to school for a couple hours a day. The remaining year and a half got me back to almost regular hours.
I still have some problems, but am able to get around most of them now because I have very acutely learned the difference between being fatigued and literal physical exhaustion. And I've become extremely patient / stubborn in my progress - a few years to make a small improvement isn't all that long any more. I still waste energy left and right, but it's no longer a necessity.
I am a rare exception, and I count myself lucky, not strong-willed. Getting to where I am now hasn't been entirely my doing - it has faded on its own to a fair degree.
---
So, throughout all this, I've kept watching the research a bit, and one thing sticks out like a sore thumb - the umbrella term. Studies have a hard time finding individuals, and a hard time finding ones with possibly-similar causes, because it manifests so many different ways (my brother had and has severe migraines, I've had digestive and muscle pains). And if it doesn't help everyone, it's not a cure. It's disorganized and counter-productive, and the same thing happens to almost every umbrella term in every field.
And due to all of this, we've had tons of blood tests, left several doctors who take the stance that it doesn't exist, and found a surprising amount of people with similar experiences. Doctors who can't accept things they didn't learn in school disgust me - I'm very glad we've found a couple that don't. They've been wonderful.
For my brother and I, I can say this: we don't know how to manifest positive tests for Mono using our minds. The rest of it is possible, but I find it unlikely, and neither of us were under odd stresses prior - both good at school, had friends, no real family troubles, etc. No doubt some of it is psychosomatic, but it varies by person, and there are definitely other factors in play in many cases. I very highly doubt many (most?) CFS cases are entirely psychosomatic. I have no problem with it being psychosomatic (really, it wouldn't bother me - it'd mean less wasted on tests and more on attempts), I just doubt it's the main cause.