Yoga philosophy says that what you are thinking about has a lot to do with the state that you are in. The way that your body functions is symptomatic of how you are feeling.
As our state changes, what we think about changes. From a yoga philosophy perspective, what we think about, how we breathe, the way our body is functioning, is representative of the state that we are in. Yoga says that experiences change how your system functions. If you alter the way your are breathing, you are consciously engineering your experience.