Human life, found but this one time More precious than the wish-fulfilling gem, So hard to regain and so easily lost, Is brief as a flash of lightning. Seeing this, discard worldly activity like the husk of a grain And strife day and night to take life’s essence. I, a yogi, did that myself; You, O liberation seeker, should do likewise.
Je Tsongkhapa, Song of the Stages in Spiritual Practice [Lamrim Dudon] 9, vs. 10 (Translation by Glenn Mullin)
the words of Gelek Rimpoche:
The great Tsongkhapa is trying to tell us: the body that we have today as a perfect human being, is the fundamental basis on which we can work on the spiritual path. It is the best basis. There are many reasons why Tsongkhapa says it is the best basis. In samsara, the circle of existence, there are a number of different beings. Human beings are not necessarily the most superior beings. The samsaric gods and the jealous gods are superior to humans. However, the life of a human being is the most suitable one to gain a perfect practice to reach enlightenment.
What we are really seeking through the Lamrim practice is nothing but ultimate enlightenment, the liberated stage of buddhahood. The stage of buddhahood is the ultimate goal. In order to produce our perfect future buddha, the best basis is the human body. The human body has a lot of special qualities. Of course we all know that, but there are additional qualities that these particular bodies of us enjoy and others cannot enjoy. Buddha himself stated that this body in this life has eighteen different qualities: the eight leisures and ten endowments.