Nityananda! Gauranga! Hare Krishna! The nature of the spirit soul is Sat-Cit-Ananda i.e. the soul is eternal, full of knowledge and full of bliss. In contrast, the body is temporary and full of ignorance and suffering. Because of this incompatibility between the two, the soul, who is meant to be happy in an eternal blissful spiritual atmosphere, is never happy in this temporary miserable body. The soul has to repeatedly experience birth, old age, disease and death in different bodies... Hence Shri Shankaracharya has sung:
punarapi jananam punarapi maranam,
punarapi janani jathare shayanam
iha sansaare bahu dustaare kripayaa paare paahi muraare
"O Muraari, Lord Gauranga Krishna, I am repeatedly tossed in the waves of birth and death. I am forced to be born again and again and forced to die again and again. In this way I have been occupying the wombs of millions of mothers. Please kindly shower your merciful glance upon me and deliver me from this ocean of nescience and take me to Your spiritual world where there is no birth and death."
In this way, the soul sometimes is damned to occupy a dog's body or blessed to get a human or demigod body, depending on the soul's past desires and activities. This cycle of birth and death is like an infinite unfathomable Ferris wheel where sometimes you are at the top and sometimes at the bottom. Similarly the soul keeps on taking one body after another until he becomes completely purified and becomes a surrendered devotee of God by chanting His Holy Names of Nityananda, Gauranga and Hare Krishna. (Adapted and commented by Swami Gaurangapada from an article compiled by Radheshyama dasa.)
Hence Shri Shankaracharya has sung: