Thinking about something stressful can have the same effect on your body as it would were you actually in the stressful situation.
If you can recall the last time you had a near miss when driving a car. Your body went automatically, by reflex, into stress mode. Your heart rate and blood pressure increased, maybe you sweated, had a churning sensation in your stomach and so on. After a moment or two your body calmed down and you continued on your way. Then, a few moments later thoughts of that near miss may have come into your mind again. What were your bodily reactions? You had similar responses with heart rate, stomach churning and so on.
This why stress management includes ‘choosing what to think’. When thinking of something stressful puts our bodies into stress mode, thinking of something pleasant can take our bodies out of stress mode. Choosing to think of something pleasant is an important part of stress management.