According to Shri Sai Satcharitra, what is the Importance of Human Birth?
In this wonderful universe, God has created billions (84 lacs according to Hindusastra calculation) of creatures or beings (including Gods, demigods, insects, beasts and men) inhabiting heaven, hell, earth, ocean, sky and other intermediate regions.
Of these, those creatures or souls, whose merits preponderate, go to heaven and live there till they enjoy the fruits of their actions, and when this is done, they are cast down while those souls, whose sins or demerits preponderate, go down to hell, and suffer the consequences of their misdeeds for so long a time as they deserve.
When their merits and demerits balance each other, they are born on earth as human beings, and are given a chance to work out their salvation. Ultimately when their merits and demerits both drop down (are got rid of) completely, they get their deliverance and become free.
To put the matter in a nutshell, souls get their births or transmigrations according to their deeds and intelligence (development of their minds).
Jaisairam.