SB 1.19.36

anyathā te ’vyakta-gater
 darśanaṁ naḥ kathaṁ nṛṇām
nitarāṁ mriyamāṇānāṁ
 saṁsiddhasya vanīyasaḥ
Word for word: 
anyathā — otherwise; te — your; avyakta-gateḥ — of one whose movements are invisible; darśanam — meeting; naḥ — for us; katham — how; nṛṇām — of the people; nitarām — specifically; mriyamāṇānām — of those who are about to die; saṁsiddhasya — of one who is all-perfect; vanīyasaḥ — voluntary appearance.
Translation: 
Otherwise [without being inspired by Lord Kṛṣṇa] how is it that you have voluntarily appeared here, though you are moving incognito to the common man and are not visible to us who are on the verge of death?
Purport: 

The great sage Śukadeva Gosvāmī was certainly inspired by Lord Kṛṣṇa to appear voluntarily before Mahārāja Parīkṣit, the great devotee of the Lord, just to give him the teachings of Śrīmad-Bhāgavatam. One can achieve the nucleus of the devotional service of the Lord by the mercy of the spiritual master and the Personality of Godhead. The spiritual master is the manifested representative of the Lord to help one achieve ultimate success. One who is not authorized by the Lord cannot become a spiritual master. Śrīla Śukadeva Gosvāmī is an authorized spiritual master, and thus he was inspired by the Lord to appear before Mahārāja Parīkṣit and instruct him in the teachings of Śrīmad-Bhāgavatam. One can achieve the ultimate success of going back to Godhead if he is favored by the Lord’s sending His true representative. As soon as a true representative of the Lord is met by a devotee of the Lord, the devotee is assured a guarantee for going back to Godhead just after leaving the present body. This, however, depends on the sincerity of the devotee himself. The Lord is seated in the heart of all living beings, and thus he knows very well the movements of all individual persons. As soon as the Lord finds that a particular soul is very eager to go back to Godhead, the Lord at once sends His bona fide representative. The sincere devotee is thus assured by the Lord of going back to Godhead. The conclusion is that to get the assistance and help of a bona fide spiritual master means to receive the direct help of the Lord Himself.