Stress problems are very common with many people reporting experiencing extreme levels of negative stress. When stress is considered as something that occurs repeatedly across the full lifespan, the true incidence of stress problems is much higher. Being "stressed out" is thus a universal human phenomenon that affects almost everyone.
What are we talking about when we discuss stress? Generally, most people use the word stress to refer to negative experiences that leave us feeling overwhelmed. Thinking about stress exclusively as something negative gives us a false impression of its true nature, however. Stress is a reaction to a changing, demanding environment. Properly considered, stress is really more about our capacity to handle change than it is about whether that change makes us feel good or bad. Change happens all the time, and stress is in large part what we feel when we are reacting to it.
We can define stress by saying that it involves the "set of emotional, physical, and cognitive (i.e., thought) reactions to a change." Thinking about stress as a reaction to change suggests that it is not necessarily bad, and sometimes, could even be a good thing. Some life changes such as getting a new job, moving in with a new romantic partner, or studying to master a new skill are generally considered positive and life-enhancing events, even though they can also be quite stressful. Other life changes such as losing a job or an important relationship are more negative, and also stressful.
Our experience of stress varies in intensity between high and low. How intensely stressed we feel in response to a particular event has to do with how much we need to accomplish in order to meet the demands of that situation. When we don't have to do much in order to keep up with demands, we don't experience much stress. Conversely, when we have to do a lot, we tend to feel much more stressed out.
Generally speaking, people do not like experiencing the extremes of stress. This is true for each end of the spectrum of stress intensity, both high and low. Few people enjoy the feeling of being overwhelmingly stressed in the face of great change. However, most people do not like a total absence of stress either, at least after a while. There is a word for such a condition (i.e., a lack of stress and challenge) which conveys this negative meaning: boredom. What most people tend to seek is the middle ground; a balance between a lack of stress and too much stress. They want a little challenge and excitement in life, but not so much that they feel overwhelmed by it.
A variety of events and environmental demands cause us to experience stress, including: routine hassles (such as getting the family out the door in the morning, or dealing with a difficult co-worker), one-time events that alter our lives (such as moving, marriage, childbirth, or changing jobs), and ongoing long-term demands (such as dealing with a chronic disease, or caring for a child or sick family member). Though different people may experience the same type of events, each of them will experience that event in a unique way. That is, some people are more vulnerable to becoming stressed out than others are in any given situation. An event like getting stuck in traffic might cause one person to become very stressed out while it might not affect another person much at all. Even "good" stressors such as getting married can impact individuals differently. Some people become highly anxious while others remain calm and composed.
How vulnerable you are personally to becoming stressed out depends on a variety of factors, including your biological makeup; your perception of your ability to cope with challenges; characteristics of the stressful event (e.g., the "stressor") such as it's intensity, timing, and duration; and your command of stress management skills. While some of these factors (such as your genetics and often, the characteristics of the stressor itself) are not under your direct control, some of the other factors are.