Some people appear remarkably composed under pressure.
A demanding week, unexpected problem or difficult decision may create temporary stress, but once the challenge has passed they seem able to recover and return to equilibrium relatively quickly.
For someone else, a similar experience can feel considerably more disruptive. Their thoughts may continue racing long after the event. Sleep may become more difficult. Concentration can suffer. They may feel mentally exhausted or find that relatively small additional demands suddenly become much harder to manage.
Why?
It is tempting to describe the first person as naturally "good at stress" and the second as simply less resilient.
The biology is considerably more sophisticated.
Humans vary substantially in how their bodies respond to psychological stress. Stress resilience is influenced by interactions between the brain, autonomic nervous system, hormonal stress response, previous experiences, sleep, behaviour and social environment.
Importantly, resilience does not mean that someone does not experience stress.
It describes the capacity to adapt to a challenge and recover after it.
Understanding that distinction changes the question.
Instead of asking:
"Why doesn't stress affect them?"
A more biologically useful question is:
"How effectively does their system respond, regulate and recover?"
And that brings us to one of the most important concepts in stress biology: flexibility.