CC Madhya 8.89

mayi bhaktir hi bhūtānām
amṛtatvāya kalpate
diṣṭyā yad āsīn mat-sneho
bhavatīnāṁ mad-āpanaḥ
Synonyms: 
mayi — unto Me; bhaktiḥ — devotional service; hi — certainly; bhūtānām — of all living entities; amṛtatvāya — for becoming eternal; kalpate — is meant; diṣṭyā — fortunately; yat — what; āsīt — there is; mat-snehaḥ — affection for Me; bhavatīnām — of all of you; mat-āpanaḥ — the means of getting My favor.
Translation: 
“Lord Kṛṣṇa told the gopīs, ‘The means of attaining My favor is loving service unto Me, and fortunately you are all thus engaged. Those living beings who render service unto Me are eligible to be transferred to the spiritual world and attain eternal life with knowledge and bliss.’
Purport: 

The fulfillment of human life is summarized in this verse from Śrīmad-Bhāgavatam (10.82.44). There are two important words in this verse: bhakti (devotional service) and amṛtatva (eternal life). The aim of human life is to attain the natural position of eternal life. This eternal life can be achieved only by devotional service.