Stress is actually just tension. You always need a little tension to perform / operate well. Many different situations can cause stress. A high work pressure is a fairly well-known example, but also illness or death of a loved one can cause a lot of stress. Yet not everyone is equally sensitive to stress. This has to do with the balance between the tolerance capacity you have and the burden you will have to endure. Having a high tolerance capacity, you can handle more or larger stressful situations than having a low tolerance capacity.
Everything you encounter, such as a high workload, illness, problems at work, an argument with your spouse or money problems can be seen as a burden. Tolerance capacity on the other hand is about your mental resilience: are you an optimistic person, can you handle disappointments well, do you not panic in conflict situations.
When you’re stressed, the burden you have to carry is heavier than your tolerance capacity. This imbalance creates stress.