Egoistic desires cannot be filled in their natural form, because the fulfillment of a desire annuls it, and as a result, it is no longer felt. Similarly, food reduces the sensation of hunger, and along with it, the pleasure from eating is gradually extinguished.
But because we are unable to exist without enjoyment, we are forced to constantly develop new desires so that we may fulfill them. Otherwise, we will not feel pleasure. This endless pursuit of pleasure constitutes our entire life, although the pleasure itself is impossible to achieve. Ultimately, disenchantment and emptiness cause depression and bring to drugs.
http://www.youtube.com/watch?v=r93Oayq09Lk