Our body evolved to secrete a hormone in times of stress, cortisol. It increases our heart rate and Help to release energy, among other things. It was very useful for the physical stressors that humans have faced since ancient times (fleeing from an animal or fighting someone, for example).
The problem is that We have changed these peaks of stress for chronic stress situations (worries at work, rushing, lack of sleep, etc.) and your body reacts in kind. We don't give our bodies a break, and this chronic stress is behind multiple illnesses, including depression, obesity, anxiety attacks, and more.
"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 bodies. Your body doesn't know if you want to win an Olympics or if you're running to avoid being eaten. Therefore, it tries to adapt in case it has to face a similar challenge in the future. We gradually fill "our cup" with our daily activities and stressors (personal worries, lack of sleep, training, etc.), and there comes a time when, if we do nothing, it overflows.