Maybe this is related, but I also feel happy (a noticeable mood boost), if I am mildly hungry.
I believe that searching for the biological answer alone is part of the problem.
I am convinced that a huge component is how we are ‘brainwashed’ over the years to accept certain things about life, via all the exposure to ads, shows, and other life experiences. Just as we age and grow old according to some script fed to us by society, we grow up to perceive exercise, exertion and physical stress according to some script we have in our minds.
Exercise, working out and physical labor are these unpleasant, dangerous mountains we must climb, and they are riddled with unpleasant challenges, and you are bound to fail and you can become one of the few people who – if you buy this program and stick to it – can overcome (for a period of time) all the unpleasant things about exercise, nutrition, etc. Oh, and there is an end to this temporary plan you will go on. You can do it! Overcome!
Also, when in mild pain: Grab Advil!! Get an MRI!! Complain!! Talk about it. Go to bed with the TV on and fall asleep to infomercials that tell you exercising this way for 10 minutes will get you the same abs the steroided-up model guy has, or the scrawny legs of the 17-year old girl who is on the eliptical machine and not sweating and looks ready for a photo shoot.
There is something wrong in Western Culture if you are hungry. OMG, you NEED 100% of the USRDA of vitamin R!!! EAT NOW! OMG, your legs are sore, and you have a mild headache = Take something! Something is wrong!
Being mildly hungry and a little sore/tired here and there = success. Unfortunately, this runs counter to everything you’ve ever been exposed to and works against the plethora of food and drugs and shows that follow you everywhere you go.
You odds of feeling great after working out = low. 90% of this issue is half mental.