Bhakti is the essence of Śrī Bhagavān’s svarūpa-śaktis named hlādinī and samvit. This svarūpa-śakti exists within Śrī Bhagavān as three faculties, namely hlādinī, samvit and sandhinī,but not within the jīva-śakti or bahiraṅgā-māyā-śakti.

By God’s mercy, the hlādinī and samvit faculties of this svarūpa-śakti enter into the heart of the jīva, and when his sādhana and bhajana mature, the jīva’s desires and inclinations become one with this energy and prema is attained.

Śrīmat Jīva Gosvāmipāda has written – tasyā hlādinyā eva kvāpi sarvānandātiśāyinī vṛttir nityaṁ bhakta-vṛndeṣv eva nikṣipyamāṇā bhagavat-prītyākhyayā vartate. atas tad-anubhavena śrī-bhagavān api śrīmad-bhakteṣu prīty-atiśayaṁ bhajata iti. (Prīti-Sandarbhaḥ, 65 anuḥ)

The meaning is that, in this way, the hlādinī-śakti is always present within Śrī Bhagavān, and whenever that most blissful, eternal faculty is deposited within the devotees, it is called bhagavat-prīti or love of God. Thus, when Śrī Bhagavān experiences the devotee’s love for Him, He also feels great love.

For example, if sulphur powder is dropped into mercury, even without mixing, just by their contact the forms of mercury and sulphur disappear and a new and different substance called mercury sulphate is created. The mercury and sulphur have united.

In the same way, as the result of bhajana such as hearing and chanting, the materialistic mentality of the bhakta-sādhaka joins with the sac-cid-ānanda nature of bhakti and spontaneously acquires a spiritual quality. The fusing of the sādhaka’s mental function and the hlādinī-śakti is called prema.

– Śrīmad Ananta Dāsa Bābājī Mahārāja

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brahmanda bhramite kona bhagyavan jiva
guru-krishna-prasade paya bhakti-lata-bija

In the course of traversing the universal creation of Brahma, some fortunate soul may receive the seed of bhakti-lata, the creeper of devotional service. This is all by the grace of guru and Krishna.”
(Caitanya-caritamrita, Madhya 19.151)

diksa-kale bhakta kare atma-samarpana
sei-kale krsna tare kare atma-sama

At the time of initiation, when the devotee offers himself up to the Lord, then the Lord makes him equal to Himself. He bestows to the devotee a body which is spiritual like His own so that the devotee can engage in the service of His lotus feet.
(Caitanya-caritamrita, Antya 4.192)

tatra sādhana-bhaktiḥ— kṛti-sādhyā bhavet sādhya-bhāvā sā sādhanābhidhā |
nitya-siddhasya bhāvasya prākaṭyaṁ hṛdi sādhyatā ||
|| Bhakti Rasamrita Sindhu 1.2.2 ||

“Action of the senses, which produces the stage of bhava, is called sadhana-bhakti. This attained state of bhava-bhakti (sadhyata) is an eternal sthayi-bhava which is not created, but simply manifests within the soul by the spiritual energy of the Lord.”

Srila Visvanatha Cakravarti Thakura explains:

The emotions and actions of bhava are all eternal and spiritual and not material because their appearance in the future will be accomplished by the extraordinary actions of the most excellent transformations (samvit and hladini) of the Lord’s svarupa-sakti.