Breath happens automatically. It is one of the parts of Autonomic Nervous System, responsible for oversight over the processes such as blood pressure, circulation, heartbeat, and breathing. Because we can’t survive without oxygen, breath is indispensable, the life force that sustains our body. That’s why it operates on an autonomous level.
Breath can be an indicator of what is happening in your body, when the stress is coming and when you start reacting to a disturbing emotion. Do you remember that your body reacts faster to an emotion that you consciously realize this? The manifestation of a difficult emotion causing you stress can also be spotted in the breathing pattern, and even faster than in other parts of your body. Just notice, how your breath accelerates or how you feel out of breath when you are scared or aroused. How it becomes faster after physical activity (which is actually is a form of stress, but a good one).