He was very fond of his youth whiling away his time enjoying all the worldly pleasures.As time went by,he started aging,but his desires to fulfill his wants never seized.He wanted another youth to fulfill the incomplete desire.
He approached his teacher Sukracharya and asked him to grant another spell of Youth.
The teacher said he did not have the power to grant him his wish.On lot of pleading, Sukracharya agreed to make Yaathi young again provided he finds some body to trade his youth with yayathi for his old age.
Yayathi could not find any body who wanted to part with his youth.So finally he asked his own son for his youth and his son due to respect agreed to the wish.
Yayathi once again became young and stared enjoying the pleasures again. But eventually,he grew old and that is when he realized the profound truth that one can never fulfill all his desires by giving into them just like a fire which cannot be put off by feeding fuel.