Thanks for your thoughtful questions, Bella. I don't have the definitive answer to why we experience exercise as unpleasnat once it exceeds a certain intensity. I can only give you an educated guess.

Exercise is typically perceived as unpleasant once we cross the ventilatory threshold (VT). VT is the point where the breathing frequency starts to increases at a faster rate than oxygen consumption. The increased ventilation is required to expel an increased amount of carbon dioxide produced due to a switch in energy metabolism from aerobic to anaerobic mode. That is also the point where lactic acid accumulates. Consider this the point at which your body is telling you that now you are working out at an intensity that is not sustainable or only for a relatively short period. If we didn’t have this unpleasant sensation, we would continue to exercise until we collapse. That must have an evolutionary background, because as the geneticist Theodosius Dobzhansky said “nothing in biology makes sense except in the light of evolution.” I see it as an ideal and welcome benchmark for training. It helps us to push ourselves into levels of fitness that better serve our health. I certainly don’t subscribe to the idea that you should exercise only because scientists say so. They say so because the evidence for the unfit-unhealthy association are overwhelming. That’s why we need to exercise because our body demands it, not because science suggests it.