SB 9.18.43

śrī-pūrur uvāca
ko nu loke manuṣyendra
 pitur ātma-kṛtaḥ pumān
pratikartuṁ kṣamo yasya
 prasādād vindate param
Synonyms: 
śrī-pūruḥ uvāca — Pūru said; kaḥ — what; nu — indeed; loke — in this world; manuṣya-indra — O Your Majesty, best of human beings; pituḥ — the father; ātma-kṛtaḥ — who has given this body; pumān — a person; pratikartum — to repay; kṣamaḥ — is able; yasya — of whom; prasādāt — by the mercy; vindate — one enjoys; param — superior life.
Translation: 
Pūru replied: O Your Majesty, who in this world can repay his debt to his father? By the mercy of one’s father, one gets the human form of life, which can enable one to become an associate of the Supreme Lord.
Purport: 

The father gives the seed of the body, and this seed gradually grows and develops until one ultimately attains the developed human body, with consciousness higher than that of the animals. In the human body one can be elevated to the higher planets, and, furthermore, if one cultivates Kṛṣṇa consciousness, one can return home, back to Godhead. This important human body is obtained by the grace of the father, and therefore everyone is indebted to his father. Of course, in other lives one also gets a father and mother; even cats and dogs have fathers and mothers. But in the human form of life the father and mother can award their son the greatest benediction by teaching him to become a devotee. When one becomes a devotee, he achieves the greatest benediction because he completely averts the repetition of birth and death. Therefore the father who trains his child in Kṛṣṇa consciousness is the most benevolent father in this world. It is said:

janame janame sabe pitā mātā pāya
 kṛṣṇa guru nahi mile bhaja hari ei

Everyone gets a father and mother, but if one gets the benediction of Kṛṣṇa and guru, he can conquer material nature and return home, back to Godhead.