Stress tends to increase during periods of rapid change. Most of the time, people succeed naturally at managing the stress in their lives. Almost everyone develops a balanced way to respond to the causes and consequences of stress. Sometimes, though, various factors make it difficult to maintain this balance. Too many emotional and intellectual demands can start to outweigh anyone's natural ability to manage stress.
Unanticipated challenges can be especially difficult.
Prolonged exposure to stress can become destructive. If the difficulties extend over a long period of time, trying to remain stoic and independent can lose its effectiveness. Getting a handle on a year-long major grind of a course is a whole other thing than making it through next week's dreaded exam. Coping with a chronically ill child can be quite different from responding to an acutely ill child. Preparing for a possible shift in the nature of your job is not the same as wrestling with a month-long project where things are going wrong.