Any efforts which exceed a person's level of tolerance can bring on stress.
Exercise is stressful for the sedentary, but is easily tolerated by physically trained people.
Likewise, socializing may be very stressful for shy person, but fun for an extrovert. Heart rate and respiration, for example, are increased by the SNS, while intestinal functions are decreased. Due to our stressful environment, the SNS is usually overstimulated. Among the severe results of sustained stress are cardiovascular diseases and immune system depression. Stress itself is a potent releaser of endorphins.