Laws of science of spirituality state that in course of its progressive evolution, the Soul of a being inhabits bodies of successively higher, more evolved species of biological kingdom and birth as a human being is the culmination of this evolution.
Vices like lust, anger, greed, infatuation and cravings for sensory pleasure keep man always entangled in the cobweb of misery. On account of the ever-changing nature of material world, one can never have lasting peace and happiness on this plane. Throughout life man keeps on lamenting for loss, craving for more and more of what he considers pleasant and getting frustrated on denial of his wishes. The attachment to the pleasant and repulsion from the unpleasant are two root causes of retrogression, which drag the being to births in sub-human species of animal Kingdom and sets the clock of progressive evolution backwards.