The breath is like a mirror for the mind. When there’s greed, anger, delusion, they’ll show up in the breath. And you find that not only does the breath reflect the mind, but you can use the breath to have a positive effect on the mind as well.
The mind is like an animal: that if it hasn’t been trained it’s difficult to live with. Once we train it, though, it stops creating so much suffering for itself. So we begin by staying in one place with something really simple: the breath.