While we remain involved in the action of the Gunas of Nature, we remain enmeshed in the ego-sense, the sense mind and the action of the senses. This leaves us subject to the force of desire, and all of the consequences that come with it, attraction and repulsion, frustration, anger, and the unsteady, uncertain and transitory joy of the attainment of an object of desire. Buddhism defines this as a state of suffering and one can easily understand why!
The practice recommended by the Gita for stilling them mind is intermediate to the migration of the entire viewpoint and standpoint being able to break away from the ego-consciousness involved in Nature, which provides the complete release from the stress and suffering of the lower nature.
Sri Aurobindo takes up the discussion: “This peace of Nirvana is reached when all the mental consciousness is perfectly controlled and liberated from desire and remains still in the Self, when, motionless like the light of a lamp in a windless place, it ceases from its restless action, shut in from its outward motion, and by the silence and stillness of the mind the Self is seen within, not disfigured as in the mind, but in the Self, seen, not as it is mistranslated falsely or partially by the mind and represented to us through the ego, but self-perceived by the Self….Then the soul is satisfied and knows its own true and exceeding bliss, not that untranquil happiness which is the portion of the mind and the senses, but an inner and serene felicity in which it is safe from the mind’s perturbations and can no longer fall away from the spiritual truth of its being.”
“The firm winning of this inalienable spiritual bliss is Yoga, it is the divine union; it is the greatest of all gains and the treasure beside which all others lose their value. Therefore is this Yoga to be resolutely practiced without yielding to any discouragement by difficulty or failure until the release, until the bliss of Nirvana is secured as an eternal possession.”
Sri Aurobindo, Essays on the Gita, First Series, Chapter 23, Nirvana and Works in the World, pp. 230-231