There's a source, there's a perennial source from which all things come forth. We call it the Godhead, Nirvana, the Tao, enlightenment. In sanskrit they say: "Tat twam asi" - thou art that. You are God.
Yoga is the science of consciousness. Yoga is a method of unifying the energies of the body, mind, and spirit and directing them towards the planes of light/nirvana.
Pleasure, pain, birth and death, lifetime after lifetime, it is endless. All sentient beings experience the endless dance of life.
In the Upanishads they talk about the path of the sun and the path of the moon. The path of the moon is rebirth. The path of the sun leads to knowledge of the Self, from which there is no return.
"Greater than the greatest, smaller than the smallest, the Self dwells in the hearts of all," the Upanishads.
In order to create heat, you have to have something to burn. So what we do in yoga is, we burn ourselves up to create light. In yoga we go to the cause. The cause of pain is not the world. The cause of pain is us.
Yoga means we accept responsibility for the tasks in our life. We know that being a king, being an enlightened teacher, being someone who sweeps the streets, we know that nothing is a greater yoga than anything else.
Sri Krishna in the Bhagavad Gita says whatever role we have in life, we have to play it to the hilt. We have to take it all the way. We have to assume responsibility for our role. To run away from it causes misery.
The message of yoga is not to become a slave to success or failure, family, friends, possessions. If these things come to you in life, enjoy them, but fix your consciousness on that which does not change - the source/nirvana/god.