Agreed. It's very difficult and definitely a learned skill to even be able to tell why or how your body/mind feels different day to day. You always feel the big things "I feel great!" "I feel like shit." but really digging down into "I am slightly annoyed right now" or "I am frustrated and cannot focus" is more nuanced.
As I have grown older and have been fortunate enough to increase stability in my life I have had more time to focus on my own health and state of mind. Only recently have I began to realize what is going on and possible causes (dehydration, lack of sleep, overtraining, etc). This is just happening now after two years of consistent (3-6 days a week) training (brazilian jiu jitsu, rock climbing, or powerlifting)
Doesn't say much.
You could have had excellent physical condition, good diet, etc before you're started exercizing and be young to begin with.