Our body evolved to secrete a hormone in times of stress, cortisol . It increases our heart rate and helps release energy among other things. It was very useful for the physical stressors that the human species has faced since ancient times (running away from an animal or fighting someone, for example).
The problem is that we have changed these stress peaks for chronic stress situations (worries at work, rush, lack of sleep, etc.) and your body reacts in the same way. We do not give our body a break and this chronic stress is what is behind multiple pathologies among others: depression, obesity, anxiety attacks...
"The time to relax is when you don't have time for it" - Jim Goodwin
Your body is able to adapt to a certain amount of stress , and sports and physical challenges are also challenges for our body. Your body doesn't know if you want to win an Olympics or if you're running to avoid being eaten. An attempt is therefore made to adapt in case in the future it has to face a similar challenge. Little by little we fill "our glass" with our activities and stressors during the day (personal worries, lack of sleep, training...) and there is a moment when if we do nothing it overflows.