When our body encounters stress, among many changes that take place in our body, one noticeable change that happens is the way we breathe. It becomes more shallow and fast. Similarly, when we are relaxed, our breathing is deeper and slower. Just like each action has an equal and opposite reaction, by becoming aware of our breath and taking slow, deep breaths whenever we encounter stress can relax our mind and body significantly.
Stress is just the opposite of relaxation. When you are relaxed, you cannot get stressed.