The Upanishads teach that we are not just body and mind, but are essentially divine. Divinity is the very basis of our existence and is the source of infinite Knowledge and Bliss. We are unaware of our divine nature because of ignorance (avidya). Ignorant of the fact that infinite Bliss is within us, we desire (kama) happiness from others and our possessions. We also desire wealth, progeny and, above all, name and fame. Desires goad us on to action (karma) for their fulfillment. Every desire-prompted action increases our ignorance. In his commentaries on the Upanishads Sri Shankara often refers to this continually revolving cycle of avidya, kama, and karma.
Our search for happiness in the external world is through five perceptions: hearing, touch, sight, taste, and smell. And the instruments for these perceptions are our five sense organs: ears, skin, eyes, tongue, and nose. The sense organs are so constituted that they are ever outward directed in order to come in touch with their respective sense objects.
Life in the world is beset with dualities: pleasure and pain, praise and blame, heat and cold, and so on. Pleasure unmixed with pain is impossible in the world. We seek pleasure, and pain comes in uninvited. In the words of Swami Vivekananda, "Happiness presents itself before man, wearing the crown of sorrow on its head. He who welcomes it must also welcome sorrow."
Buddha realized long ago that desire is the cause of all misery, and proclaimed it as one of his Four Noble Truths. A life of unbridled sense enjoyment is bound to end up in misery and frustration. The Chandogya Upanishad (7.23.1) teaches the important truth that infinite bliss is not possible from finite things: "The Infinite is bliss. There is no bliss in anything finite. Only the Infinite is bliss. One must desire to understand the Infinite."
In the Bhagavad Gita (3.36-37), Arjuna asks Sri Krishna an important question: "Under what compulsion does man commit sin, in spite of himself and dragged, as it were, by force?" Replies the Lord, "It is desire, it is anger; both spring from rajas. These are our enemies, all-devouring and the cause of all sin." A poignant verse describes how Duryodhana became helpless when he was overpowered by desire for his cousins' land and kingdom: "I know what is righteousness, but I cannot practice it; I know what is unrighteousness, but I cannot refrain from it." (Pandava Gita, 58) Desire and anger are like twin brothers. And in the words of the Gita (16.21), when greed joins them they form a triple gateway to hell.
People think that they will have their full share of worldly enjoyment now, and in the evening of life the mind will become desireless and calm, and inclined to study and spiritual practice. But it never happens that way. Our mind that has been attached to worldly enjoyment for a long time cannot suddenly turn to higher things because we have become free from worldly responsibilities. Also, enjoyment does not make the mind calm and desireless.
Desires only grow stronger with fulfillment. King Yayati's life from the Bhagavata illustrates this truth. When Yayati was in the prime of his youth a sage, incensed by the king's behavior, cursed him to premature old age. The king begged the sage's pardon and prayed for a remedy. The sage said that the king could remain young if someone exchanged his youth for the king's old age. The king's youngest son willingly exchanged his youth for his father's old age and the king enjoyed sense pleasures for innumerable years. If desires could be quenched through enjoyment, Yayati would have become calm and sated after so much enjoyment. Instead, he discovered a profound truth: "Desire can never be quenched by enjoying sense objects. Like fire fed with clarified butter, it only flames up all the more." (Bhagavata, 9.9.14)