But this material world is an endless, unpredictable mix of pleasant and unpleasant circumstances that we can't control. When we let our mind's decisions—based on our desire for temporary pleasure—dictate our choices, our happiness isn't guaranteed at all. A materially oriented mind can't lead us to permanent fulfillment. For that, we need to train the mind to make judgments based on a more elevated worldview.
The system of yoga, as Krishna explains in the Bhagavad-gita, is meant to make our life peaceful by giving our mind a superior objective—specifically, meditating on the Supreme Person. Scriptures say that this, Krishna consciousness, is the perfection of all mental activity. When the mind is controlled in this way, it can be our best friend and help lead us to genuine happiness. Then we can be undisturbed by the temporary comings and goings of pleasure and pain in this world.