What is the difference between negative and healthy stress?

In 1975 Hans Selye published a model that divided stress into eustress (good stress) and distress (bad or negative stress). Selye argued that where stress enhances human functioning and facilitates better performance it should be considered good stress or eustress. He argued that good stress enables you to rise to life’s challenges and achieve optimal performance. Beyond a certain point, however, stress stops being helpful and starts causing major damage to your health, your mood, your productivity, your relationships, and your quality of life. This is what Selye referred to as distress or negative stress. The difference between events that result in eustress or distress is determined by a person’s ability to appraise a situation as a challenge rather than as a threat and, crucially, their coping mechanisms that may include exercise.

Living as a human being is stressful - our lives are fill of stressors - or anything that puts a strain on our system.  Some of these stressors are more controllable than others, and some are more pleasurable than others.  For example, a very busy demanding job is full of stressors, but if that job fills us with a sense of purpose or meaning, then the stress may be healthy. In addition, stress is very much about controllability - if we have a lot of stress in our life yet we feel we can manage it - either by being able to manage the stressor directly or have ways to manage the stress (e.g. exercise, social support) that we can control - then we can take a healthy approach to stress.  Stressors and stress over which we feel little control, that lead to wear and tear on our bodies, and that translate into cognitive states such as hopelessness or helplessness tend to be classified as "negative."  Controllability and coping are key elements to understanding the difference between negative and healthy stress.

The same event or situation may produce healthy stress for one person and negative stress for another person, much of which is determined by the way that person perceives it. For instance, if one person gets fired, he may think it is the worst thing that could have happened, and focus on all the negative consequences that may happen, becoming tense, fearful, negative, and even depressed. Another person may believe that while being fired is unfortunate, it can also be an opportunity for further training, an opportunity to move to a region of the country he prefers, or to seek employment in another field he has always been interested in. He is likely to experience healthy stress which motivates him to take action and make decisions in his best interest. The way a person thinks and perceives has a big effect on how the body reacts to stress, and much of that is a choice.