Although Śrīman Mahāprabhu is the jagad-guru (universal teacher), He still accepted Śrī Īśvara Purī as His guru. In this way, the Lord taught the sādhakas of the world by example, that the lotus feet of the Lord can`t be attained without taking shelter of a guru.

In Caitanya Bhāgavata it is said –
śikṣā guru īśvara ye korāyen śikṣā; ihā ye mānaye sei jana pāya rakṣā:

“A person who follows the teachings of an instructing spiritual master, who is like God Himself, will be redeemed.”

The guru is the boatman, who helps the fallen conditioned souls to cross over the ocean of material existence.

In Śrīmad Bhāgavata Śrī Kṛṣṇa says:

nṛ deham ādyaṁ sulabhaṁ sudurlabhaṁ
plavaṁ sukalpaṁ guru karṇa-dhāram
mayānukūlena nabhasvateritaṁ
pumān bhavābdhiṁ na taret sa ātmahā

“This human body is a great asset that is rarely attained. He who does not use this body as a vessel to cross over the ocean of material existence, even though it may be steered by an expert guru and pushed on by the favorable winds of My mercy, is committing suicide.”

The guru is the embodiment of the Lord’s mercy, who descends to the world for the benefit of the conditioned souls. When the Lord met Īśvara Purī at Gayā, He said:

saṁsāra samudra hoite uddhāro āmāre;
ei āmi deha samarpilām tomāre
kṛṣṇa pāda padmera amṛta-rasa pāna;
āmāre korāo tumi ei cāi dāna

“Please lift Me up from the ocean of material existence; I have given My body to you. Please bless Me by making Me drink the nectar of Kṛṣṇa’s lotus feet! This is the gift I want!” The night before, Īśvara Puri already had a dream in which he understood who the Lord actually was. As he showered the Lord with a stream of the love-honey from his heart, he said:

bolena īśvara purī śunaho paṇḍita;
tumi ye īśvara aṁśa jāniluṁ niścita
ye tomāra pāṇḍitya ye caritra tomāra;
eho ki īśvara aṁśa bohi hoy āra
yeno āji āmi śubha svapna dekhilām;
sākṣāte tāhāra phala ei pāilām
satya kohi paṇḍita tomāra daraśane;
parānanda sukha yeno pāi anukṣaṇe
yad avadhi tomā dekhiyāchi nadīyāya;
tad avadhi citte āra kichu nāhi bhāya
satya ei kohi – ithe kichu anya nāi;
kṛṣṇa daraśana sukha tomā dekhi pāi

“Listen, O Paṇḍita! I know for sure that You are an expansion of the Supreme Lord. Who else but an expansion of the Lord could be as qualified and learned as You? Tonight I had a sacred dream and today I directly obtained its fruit. I tell you truly, O Paṇḍita, when I see You, every moment I feel most blissful! As soon as I saw You in Nadīyā, my mind could not be attracted to anyone or anything else anymore. I tell You the truth: When I see You, I feel as blissful as when I would see Kṛṣṇa Himself!”     The Lord was especially showered with the love-honey from Īśvara Purī’s heart and thus He became even sweeter. Desiring to inundate the whole world with this sweetness, One day He went to ask for mantra-initiation from Īśvara Purī:

āra dine nibhṛte īśvara purī sthāne;
mantra dīkṣā cāhilen madhura vacane
purī bole mantra vā boliyā kon kothā;
prāṇa āmi dite pāri tomāre sarvathā
tabe tāra sthāne śikṣā guru nārāyaṇa;
korilena daśākṣara mantrera grahaṇa
tabe prabhu pradakṣiṇa koriyā purīre;
prabhu bole deha āmi dilām tomāre
heno śubha dṛṣṭi tumi koroho āmāre;
āmi yeno bhāsi kṛṣṇa premera sāgare
śuniyā prabhura vākya śrī īśvara purī;
prabhure dilena āliṅgana vakṣe dhari
doṅhāra nayana jale doṅhāra śarīra;
siñcita hoilo preme – keho nahe sthira

“One day the Lord privately asked Īśvara Purī for mantra-initiation with a sweet voice. Īśvara Purī said: “What to speak of a mantra, I could give You my whole life!”
Then the instructing guru Nārāyaṇa (Nimāi) accepted the ten-syllable initiation-mantra from Īśvara Purī right there on the spot. The Lord circumambulated Īśvara Purī and said: “I have given you My body. Now cast an auspicious glance at Me, so that I may float in the ocean of love for Kṛṣṇa!”

When Śrī Īśvara Purī heard the Lord’s words, he embraced Him. They showered Each other with their tears of love and they could not calm down.

Although the Lord is the ocean and the descension of Love Himself, He still became sweeter than He was (showing even more prema than He did before), after having been showered with the love-honey coming from Īśvara Purī’s heart.