The life of a human being is a chance to prepare oneself to go back to Godhead, or to get rid of the material existence, the repetition of birth and death. Thus in the system of varnasrama-dharma every man and woman is trained for this purpose. In other words, the system of varnasrama-dharma is known also as sanatana-dharma, or the eternal occupation. The system of varnasrama-dharma prepares a man for going back to Godhead, and thus a householder is ordered to go to the forest as vanaprastha to acquire complete knowledge and then to take sannyasa prior to his inevitable death.
Srila Prabhupada, SB 1.19.4, ppt.