Your body is not able to tell the difference between eustress (good life events) and distress (stress leading to negative outcomes). So both eustress and distress trigger the fight or flight response, and both types of stress are equally taxing on your body.
The difference between eustress and distress is a matter of functioning and performance. It is eustress if the stress enhances and improves your functioning (see optimal performance on inverted-U curve below). It is distress if the stress overwhelms your ability to cope, and leads to anxiety and poor functioning (see panic attack on inverted-U curve below.
The fight or flight response is a natural response that occurs in response to a perceived threat or danger. This response is triggered by the release of hormones such as adrenaline and cortisol, which prepare the body to either fight the threat or run away from it. Eustress, which is a type of stress that is positive and motivating, can trigger this response in situations where a person is faced with a challenging or exciting situation. Distress, on the other hand, is a type of stress that is negative and overwhelming, and it can trigger the fight or flight response in response to a perceived threat or danger.