Yoga teaches that everything is real, and everything is subject to change. Our thoughts, dreams, fears are real, and are part of our life. However Yoga also teaches that suffering is caused by ignorance, and ignorance is caused by fear, anger, attraction and aversion. The practice of Yoga makes clear the relationship of mind to experience, by focusing on the breath and letting the body be, we realize that even if our body is struggling (experience), by controlling our breath and allowing the breath to be the major occupation of the mind, our mental state is that of peace. Stepping off the mat, whenever we are in a situation that threatens to create stress or tension, we can remember to connect with the breath and allow that to calm our mental state. In this simple way, Yoga helps us navigate the ups and downs of daily life with more ease.
Another effect of the focusing of the mind on the breath, is that the mind is inhibited from its usual tendency to worry about the future or dwell on past experiences, by dwelling in the breath, the mind becomes focused on the present moment, on something essential to life, this brings about the quality of Presence, of a mind that is aware of the present moment, as embodiesd in the ebb and flow of the breath. This state of presence, much sought after in mystical traditions, is a quality that can only be experienced to be understood, but brings with it a clear and unperturbed perception of events.
A third way in which Yoga leads to happiness, is that in clarifying the relationship of mental contents to experience, and in clarifying indeed the contents of mind, we come to see ordinary things in an extra ordinary way, as if for the first time, this leads to an increase in wonder at the miracle of existence and consequently awe and gratitude.