I am a very rational/scientific-type person, very modern in that sense (though I have tried to learn more about the more sacred/spiritual thinking of the past, since it probably played an important role in the evolution of human societies over at least the first 2.5 million years of human existence--not something that can be easily poo-pooed), so my responses will tend to be along those lines. I hope none of the following offends spiritual people and they may wish to skip my post if they fear it will.
The following is speculation on my part, so I'm not claiming that any of it is fact and you should take it with a grain of salt.
Lex or PaleoPhil (or anyone else who feels this... Wodgina?):
Is this feeling of 'well being' or 'euphoria' something physiologic or a mental state?
I'm not sure what the difference is between those two, and scientists have been debating for decades over how much of mental state is physiological/chemical/hormonal/etc., so I'll let them figure that out. First I'll try to explain what I meant by euphoria. Euphoria and well-being are synonyms, with euphoria generally being seen as being an enhanced state of well-being beyond the everyday norm. The Greek word euphoria literally means well-bearing, which is translated into "well being" in English. Wikipedia states: "Euphoria is medically recognized as an emotional and mental state defined as a sense of great elation and well-being." It is safer to use the term "well being," because euphoria has also become associated with the drug-induced highs that have become increasingly common and accentuated in modern society in recent centuries, but for me, "well being" didn't seem to sufficiently convey the positive feeling I experienced that exceeded anything I had felt in my prior 40 years of eating modern foods.
All I know is that it was entirely unexpected when it first happened, it took me by surprise, puzzled me, and involved no conscious effort on my part. In other words, I didn't say to myself, "Oh isn't it wonderful how healthy I'm eating, I feel so good about myself, I think I'll adopt a positive mental attitude and meditate and try to reach a higher plane of consciousness, etc." It was more like, "Let's try this WOE that others report works so well and the science seems to back, hmmm, this is a fascinating positive sensation that seems to be welling up from deep inside my gut and washing over my body, what the devil is this? Am I really experiencing this or is this some sort of coincidence or delusion--let me check with others. Well, what do you know, multiple other people report the same sorts of euphoric/good-well-being/etc.-type sensations, so there appears to be some sort of 'real' cause and effect going on here. Fascinating. This is worthy of further study and seems to confirm that I am on the right dietary track."
Lex also seems to be a very rational/scientific type of person (coincidentally, he was an engineer and I started out on an engineering track in college, though I switched to business), so when he reports similar phenomena, I take his reports very seriously. He comes across as an honest, no-nonsense sort of person who wouldn't make up stuff or be easily prone to supernatural whimsy.
Whatever it is, for me it seems to emanate more from the primordial brain (the gut) and the body than the second cranial brain, and my cranial brain seems to be a beneficiary of its effects rather than the source of them. I don't take drugs or even consume much caffeine and I am not the sort of person who is prone to having or reporting mind/consciousness-altering or supernatural experiences (when a teacher asked my high school English class who had encountered ghosts, I was one of only 3 out of about 30 students who didn't raise their hands--it is a vivid memory because I was shocked at how many thought they had had experiences with ghosts). However, this experience has made me more interested in the spiritual reports and practices of traditional peoples and I have gradually incorporated more sacred-type language into my own discourse on "sacred" matters, because I now take them more seriously and think there may more of a physiological/physical/chemical/hormonal/call-it-what-you-will basis to much of it than I realized.
It sounds like Lex's positive sensations moderated over the years and I suspect that mine probably will as well. I think that part of it involves deficiencies in minerals and other nutrients that when filled most optimally by fresh flesh--perhaps especially raw meat and fat--result in positive biofeedback from the body, which would encourage mammals like us to eat more of that healthy flesh and thus better ensure our survival and thus may be an evolutionary adaptation.
My experience seems to confirm this. For example, when I first ate a large amount of wild salmon in the early days of adopting a NeanderThin-type diet, I experienced the greatest feelings of euphoria/well-being and they were lessened on later eating of similar quantities of wild salmon. Also, they seem to be greatest when I've gone the longest without eating or after having done some cheating and then return to the foods I do best on, suggesting that my nutrients have depleted more and are now being replenished more greatly than usual.