The Curse
King Yayati had two wives: Devayani (daughter of Shukracharya, the guru of the asuras) and Sharmishtha. When Shukracharya discovered that Yayati had favored Sharmishtha, he cursed Yayati with premature old age.
Yayati begged for relief. Shukracharya relented slightly: Yayati could transfer the old age to a willing son temporarily.
The Request
Yayati went to each of his sons in turn. Each refused — they were young, they had their own lives to live.
The youngest son, Puru, agreed without hesitation. He took his father's old age into his own body. Yayati became young again.
The Thousand Years
Yayati pursued every pleasure available. He enjoyed music and beauty and wealth and love and power. For a thousand years.
At the end of a thousand years, he called Puru to him. He returned the old age. He spoke these words, now famous:
*"Desire is not quenched by enjoyment as fire is not quenched by pouring ghee. It only grows greater. I have enjoyed pleasures for a thousand years, and I say to you: desire has no end. Renounce it."*
The Return
Yayati gave his kingdom to Puru, whose line would eventually produce the Pandavas and Kauravas. He went into the forest to perform tapasya.