When you really think about it...
When I sit and take notice, I suddenly focus on my breathing. I notice the rush of coolness in the back of my throat as I pull in a long breath, and I feel the sticky dryness left when the breath has passed, causing me to swallow to moisten it.
Today I notice how all of the air seems to pass through only one side of my nose, because of the cold that I can't seem to shake. The more I think about it the more it starts to bother me, and I have to force myself to take deeper, harder breaths to make sure my entire nose is being used to its full capacity.
Now that my nose is working again, I begin to think about how natural breathing is; a reflex. I marvel at how the body never forgets to draw breath. As I think this, I start worrying that my body will forget, because I find that I've actually been holding my breath. I can feel my heart pounding just a little bit harder, and I have to concentrate for a moment to calm myself until my breathing returns to normal.
Probably better left to reflex, over analyzing my breathing only disrupts the process!