People who regularly practice any kind of sport, especially running, are familiar with a certain euphoria. This phenomenon is called “muscle joy.”
After all, a person's physical condition and emotions are closely related. And during physical activity, a complex mechanism works associated with neurohumoral activation and the production of many hormonal compounds, including hormones of joy. Therefore, despite fatigue, I want to prolong this feeling associated with physical activity.
I cannot answer exactly and medically, but I will try to clarify this point. Some time after training, the body produces some kind of happiness hormone, whose name is endorphin. I will not go into polemics, there are many difficult words, but I don’t remember them from my course in biology. Endorphin is released into our bloodstream during prolonged physical activity, and with intense training, its content in the blood can rise up to 4-5 times, in contrast to the state of the same rest. During short workouts, endorphin is also released into the flow of our life-giving scarlet liquid, but in much smaller quantities - it is clear, because the body prepares itself for prolonged physical exertion.
Endorphins affect some receptors and cause a feeling of high spirits or even euphoria.
There is another theory, there is something about marijuana, but it is controversial.