My teacher Sharon Gannon, co-founder of the Jivamukti Yoga method, says, "You cannot 'do' yoga. Yoga is your natural state. What you can do are yoga practices, which may reveal to you where you are resisting this natural state of happiness, ananda, bliss."
Why would anyone resist a state of happiness and bliss? We resist because we live with the illusion of separateness. We are attached to our bodies, our thoughts, our egos, our personalities, the roles we play and the responsibilities we have. We think all of those things are who we are. We define ourselves in terms of our individual self (with a small "s"). We get caught up in who we think we are and don't realize who we truly are. We are ignorant of our True Nature - our Higher Self (with a capital "s") which is unconditional love, boundless compassion, limitless joy, and unlimited potential.
That's where yoga practices come in. When we commit to the practices of yoga (such as asana, pranayama, meditation, kriyas, mantra, study, etc.) with with the intention of freeing ourselves from ignorance, then we will eventually but inevitably become free of deep-rooted fears, selfishness, and ignorance. We will realize the highest aim of yoga practices - the realization of the Oneness of Being, the state of Yoga.
Consider the teaching "Atman is Brahman". The Sanskrit word Atman is the individual soul. Every living being on this planet - humans, other animals, and plants - has an Atman. Every living being has prana (life-force energy) running through it. Each being has an immortal essence that lives on, even after death of the physical body. Brahman is the cosmic soul, the divine Universe, the Ultimate Reality of the cosmos. Brahman is pure consciousness, the Absolute Reality of existence. It cannot be described - it can only be experienced. "Atman is Brahman" means that the individual soul is no different than the Universal soul. The microcosm is the macrocosm. Our sense of separateness is an illusion. We are not only One with every other living being on the planet, we are also One with the divine Universe, with Brahman. Brahman is within each living being.
Astrophysicist Neil deGrasse Tyson echoes this teaching of yoga when he says that, "We are part of this universe; we are in this universe, but perhaps more important than both of those facts, is that the universe is in us." And astronomer Carl Sagan reminds us that the macrocosm is in the microcosm as well, "The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of star stuff." Long before the time of Tyson and Sagan, between 800-400 BCE, sages composed the sacred scriptures known as the Upanishads. The concept that "Atman is Brahman" pervades those texts. In the Mandukya Upanishad it is written, "OM. This eternal Word is all: what was, what is and what shall be, and what beyond is in eternity. All is OM. Brahman is all and Atman is Brahman."