Srimad-Bhagavatam: Canto 11 - Chapter 1 - Verse 29

Sanskrit:

लब्ध्वा सुदुर्लभमिदं बहुसम्भवान्तेमानुष्यमर्थदमनित्यमपीह धीर: ।तूर्णं यतेत न पतेदनुमृत्यु याव-न्नि:श्रेयसाय विषय: खलु सर्वत: स्यात् ॥ २९ ॥

ITRANS:

labdhvā su-durlabham idaṁ bahu-sambhavāntemānuṣyam artha-dam anityam apīha dhīraḥtūrṇaṁ yateta na pated anu-mṛtyu yāvanniḥśreyasāya viṣayaḥ khalu sarvataḥ syāt

Translation:

After many, many births and deaths one achieves the rare human form of life, which, although temporary, affords one the opportunity to attain the highest perfection. Thus a sober human being should quickly endeavor for the ultimate perfection of life as long as his body, which is always subject to death, has not fallen down and died. After all, sense gratification is available even in the most abominable species of life, whereas Kṛṣṇa consciousness is possible only for a human being.

Purport:

Material life essentially means repeated birth and death. Even the lowest forms of life, such as reptiles, insects, pigs and dogs, have ample opportunity for sense gratification. Even ordinary houseflies have a busy sex life and thus multiply rapidly. Human life, however, enables one to understand the Absolute Truth and is therefore full of grave responsibility. Since the valuable human life is not eternal, we should do the needful to achieve the highest perfection, Kṛṣṇa consciousness. Before death appears, we should seriously cultivate our real self-interest. One can experience Kṛṣṇa consciousness in the association of devotees of the Lord. Without their association, one is in danger of being attracted to an impersonal conception of life, which causes one to fall away from devotional service to the Absolute Truth. Or, being discouraged by one’s failure to understand the Absolute Truth, one may return to the false platform of sense gratification. In conclusion, human life is meant for cultivating Kṛṣṇa consciousness under the guidance of the experienced, self-realized devotees of the Lord.