In medicine there is a saying, "You can live three weeks without food, three days without water and three minutes without air." Breathing is so natural and automatic that unless there is something preventing you from doing it, you never notice it happening. But in those times when you can't breath, you notice immediately and only have a couple minutes to correct the situation, or you die.
But why do you need to breathe constantly in order to stay alive? Most people are aware that we breath to get oxygen. If you didn't know that, you need to start reading more. A lot more.
Oxygen is a vital element in human metabolism. Oxygen is used for the various processes of cellular respiration. Cellular respiration is a series of chemical reactions by which your cells breakdown glucose, as well as amino acids and other high energy compounds. Without oxygen, your body would rapidly loose the ability obtain the energy needed to sustain life.
Under normal circumstances people breath automatically. You don't have to make a conscious effort to breath. There is some ability to override the natural urge, but only for a limited time. Try breathing faster than normal for a few seconds and you'll see when I mean. Conversely, try holding your breath - you can only do it for a short time before the natural control steps in and takes over.
Interestingly, the primary drive to breath is normally not related to oxygen. The amount you breath each minute is closely related to the levels of carbon dioxide in your blood. More carbon dioxide in your blood lowers the blood acidity, which leads to increased rate of breathing. As the breathing rate increases, you blow off the excess carbon dioxide and the blood acidity returns to normal. Carbon dioxide is a waste product of cellular respiration, so the more energy you burn, the more carbon dioxide you create, and the faster you breathe. Run around the block and you'll get the idea.
Breathing correctly is easy to take for granted until you are no longer able to. Asthmatics, people with emphysema or chronic bronchitis and even acute infections can keep a person from breathing properly. These problems need to be taken seriously and corrected as soon as possible. If something were to go really wrong, you only have three minutes, give or take a bit, before you find yourself in a world of trouble.
As my physiology professor in medical school said, "Everyone stops breathing eventually." Nice cheery thought, eh?