Respiration (simply breathing) isn't something we have to think all the time. It's automatic most of the time but it's controlable to a limited amount. I read about respiration from Galenos (the entrance examination book of medicine in Finland) today. Why do we breath? I try to give you a little briefing what I remember.
We breath so that we can get oxygen, which massively boosts our metabolism. In other words: we get much more energy if we use oxygen. On the other hand, we breath so that we can get rid of carbon dioxide and other metabolic waste. Brains are very dependent on oxygen: if a brain-leading artery gets blocked, only a pause of 5 seconds in oxygen transmission can cause senselessness.
How come the air flows into our lungs? By breathing yeah, but we can't just grab those molecules in. The answer is: under pressure. As the diaphragm (the main inspiratory muscle) contracts, it expands the volume of lungs, which creates under pressure inside the lungs and the air flows from higher pressure (outside) to lower pressure (lungs).
Remember to breath folks! ;) It's essential!