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SB 3.2.15

sva-śānta-rūpeṣv itaraiḥ sva-rūpair
 abhyardyamāneṣv anukampitātmā
parāvareśo mahad-aṁśa-yukto
 hy ajo ’pi jāto bhagavān yathāgniḥ
Synonyms: 
sva-śānta-rūpeṣu — unto the peaceful devotees of the Lord; itaraiḥ — others, nondevotees; sva-rūpaiḥ — according to their own modes of nature; abhyardyamāneṣu — being harassed by; anukampita-ātmā — the all-compassionate Lord; para-avara — spiritual and material; īśaḥ — controller; mahat-aṁśa-yuktaḥ — accompanied by the plenary portion of mahat-tattva; hi — certainly; ajaḥ — the unborn; api — although; jātaḥ — is born; bhagavān — the Personality of Godhead; yathā — as if; agniḥ — the fire.
Translation: 
The Personality of Godhead, the all-compassionate controller of both the spiritual and material creations, is unborn, but when there is friction between His peaceful devotees and persons who are in the material modes of nature, He takes birth just like fire, accompanied by the mahat-tattva.
Purport: 

The devotees of the Lord are by nature peaceful because they have no material hankering. A liberated soul has no hankering, and therefore he has no lamentation. One who wants to possess also laments when he loses his possession. Devotees have no hankerings for material possessions and no hankerings for spiritual salvation. They are situated in the transcendental loving service of the Lord as a matter of duty, and they do not mind where they are or how they have to act. Karmīs, jñānīs and yogīs all hanker to possess some material or spiritual assets. Karmīs want material possessions, jñānīs and yogīs want spiritual possessions, but devotees do not want any material or spiritual assets. They want only to serve the Lord anywhere in the material or spiritual worlds that the Lord desires, and the Lord is always specifically compassionate towards such devotees.

The karmīs, jñānīs and yogīs have their particular mentalities in the modes of nature, and therefore they are called itara or nondevotees. These itaras, including even the yogīs, sometimes harass the devotees of the Lord. Durvāsā Muni, a great yogī, harassed Mahārāja Ambarīṣa because the latter was a great devotee of the Lord. And the great karmī and jñānī Hiraṇyakaśipu even harassed his own Vaiṣṇava son, Prahlāda Mahārāja. There are many instances of such harassment of the peaceful devotees of the Lord by the itaras. When such friction takes place, the Lord, out of His great compassion towards His pure devotees, appears in person, accompanied by His plenary portions controlling the mahat-tattva.

The Lord is everywhere, in both the material and spiritual domains, and He appears for the sake of His devotees when there is friction between His devotee and the nondevotee. As electricity is generated by friction of matter anywhere and everywhere, the Lord, being all-pervading, appears because of the friction of devotees and nondevotees. When Lord Kṛṣṇa appears on a mission, all His plenary portions accompany Him. When He appeared as the son of Vasudeva, there were differences of opinion about His incarnation. Some said, “He is the Supreme Personality of Godhead.” Some said, “He is an incarnation of Nārāyaṇa,” and others said, “He is the incarnation of Kṣīrodakaśāyī Viṣṇu.” But actually He is the original Supreme Personality of Godhead — kṛṣṇas tu bhagavān svayam — and Nārāyaṇa, the puruṣas and all other incarnations accompany Him to function as different parts of His pastimes. Mahad-aṁśa-yuktaḥ indicates that He is accompanied by the puruṣas, who create the mahat-tanva. It is confirmed in the Vedic hymns, mahāntaṁ vibhum ātmānam.

Lord Kṛṣṇa appeared, just like electricity, when there was friction between Kaṁsa and Vasudeva and Ugrasena. Vasudeva and Ugrasena were the Lord’s devotees, and Kaṁsa, a representative of the karmīs and jñānīs, was a nondevotee. Kṛṣṇa, as He is, is compared to the sun. He first appeared from the ocean of the womb of Devakī, and gradually He satisfied the inhabitants of the places surrounding Mathurā, just as the sun enlivens the lotus flower in the morning. After gradually rising to the meridian of Dvārakā, the Lord set like the sun, placing everything in darkness, as described by Uddhava.