I was once told by a teacher of mine that the only thing you can control in your life is your breath. The longer I am a yogi and a seeker the more clear and evident this becomes.
What becomes even more clear is the fact that your breathing can effect the entire world around you. When we aren't breathing properly our bodies react as if we are in a stressed state. Even though, we may not actually be. This can lead to tension in the chest, shoulders and neck because we are breathing into the ribs and not the belly. We are actively expanding the chest instead of using the diaphragm correctly. This alone can also lead to an increase in unnecessary stress. It effects our basic physiology so much that the mind thinks we are in a stressful situation even though we are not. Then the mind simply follows what the body is implying.
Sure, you may think "But my lungs are in my chest," but the diaphragm is at the base of the rib cage. Once we take control of the diaphragm and breath deeper into the base of our lungs our oxygen supply increases. Thus, relaxing the body and again, the mind follows.
When the body is relaxed, so is the mind. That's why breathing is so fundamental to your yoga practice and basically life in general. It's the most basic thing we need to survive but somehow we always forget how to do it properly.
Do me a favor and just sit up straight, place your hands on your belly and try to breath into them while keeping your eyes closed. Take 10 really deep, long, slow breaths in and out. I'll wait....
Now how do you feel? You should clearly notice the effect it has had on your physical and mental state. Thats why its so important to breath deeply.
Now that you are more relaxed in body and mind your decisions and actions will change. Thats exactly how breath can change your world. You will now respond and react better to the daily stresses of life instead of holding on to them and letting them effect your mental and physical state if you practice this exercise any time you are feeling stressed.
This simple technique is a way to help us begin to take back control of our mind and our lives.
Try to do it every time you feel like you are tense and see how much it changes things.