Any member of the species that was more laid back would leave fewer off-spring and hence disappear from the gene pool. So, he concludes, desire is built-in and natural. I think he's right. But key here is the fact that one does not have to be happy or contented to breed successfully. Desire increases the probability of breeding success, but not happiness.
Desire is incessant and insatiable, more or less.
But since desire is boundless, so will be our search for what will satisfy it. Millions of people in a desert jamming sand down their throats hoping that this will at last be water. Not only are they not important, but satisfying them will not bring you happiness or even contentment. Not only that, but satisfying them might actually do harm and bring you pain. Not only that, but your desires are actually the source of your suffering.