When Arjun asks Lord Krishna what is it that drives men to commit sin, at times even involuntarily, Lord Krishna replies:
Kaam esha krodha esha rajogunasamuddhavah
Mahaashano mahaapaapmaa viddhyenanih vairinam
— Bhagavad Gita, Chapter 3, Verse 37 & 38
It is desire, which is anger, which is born of rajas guna. It is insatiable and pure evil. It is the greatest enemy of man in this regard. Just like smoke covers fire and dust covers a mirror, preventing one from seeing the light, the same way gyan is clouded by desire, which is unending.
It is desire that binds us to unreality, the body and the five elements, which are temporary and bound to leave. The interesting thing about desire is that it is insatiable. Mahabharat narrates the story of King Yayati in this regard. King Yayati was approaching old age but his desire for sensory pleasures was not satiated. He exchanged his old age for the youth of his son Puru, and continued indulging in all kinds of pleasures for another 1,000 years. Even after thousand more years of all this, he realised nothing had changed, and he was still dissatisfied and wanted more...
While one spends life and birth chasing desires, the desire never ends, the senses dull, body shrivels away and the reality or gyan eludes. The seat of desire is the senses, the mind and the intellect and so one needs to develop control over senses to overcome desires, which destroys gyan. This is achieved through practice of yog under a guru. Once the senses are stilled, certain centres open up, bestowing amazing abilities and experiences of gods and goddesses.
Let us do a simple experiment for a first-hand experience. Sit with your eyes closed and watch your breath at the tip of the nostrils. Now, take a deep breath and holding the breath, use your thumb and fingers to block the seven doors to external world — ears with thumb, eyes with index fingers, nose with middle fingers and mouth with ring and little fingers. Stay for as long as comfortable. Exhale.