There are many things that can cause someone to take shallower breaths, but the most common reason is stress. When we’re stressed, our body goes into fight-or-flight mode and starts to breathe more shallowly in order to get more oxygen to our muscles so we can either fight or run away.
This is an evolutionary response that served us well when we were constantly being chased by predators, but these days most of the things that stress us out are not life-threatening. Unfortunately, this means that our bodies are often in a state of high alert even when there’s no real danger, which can lead to all sorts of health problems.
Shallow breathing can also be caused by certain medical conditions such as asthma, COPD, and heart failure. In these cases, the shallow breathing is a symptom of the underlying condition and not the cause.