Hans Selye originally defined stress as the body's response to challenges. He was dismayed by the implication that all challenging events in life were unhealthy and undesirable. Stress was not always bad, he pointed out. Sometimes a challenge is a good thing. Indeed, one could argue that nothing useful in life can be accomplished without some degree of stress.
"Good stress," Selye pointed out, is "the spice of life." To combat the notion that all stress was bad, Selye developed the idea of eustress, which is a person's ideal stress level. Selye proposed that different people needed different levels of challenge or stimulation (stress) in their lives. Some people ("turtles") need low levels of stress. Others ("racehorses") thrive on challenges.
In the long run, the popular conception of stress as something bad proved to be more durable and accurate than Selye's notion of stress as a challenge to the system. In other words, the word stress continues to mean something bad (not something challenging) to most people. That seems to make the most sense, because psychologists found that only unpleasant stressors produced the harmful stress reaction identified by Selye (corticosteroid secretion). Challenges were not harmful in themselves. A person could be a busy executive or engage in strenuous exercise without experiencing negative stressrelated symptoms, as long as the person enjoyed the challenge.