Srimad Bhagavatam Canto 03, Chapter 02, Text 15

SB 3.2.15

sva-santa-rupesv itaraih sva-rupair
 abhyardyamanesv anukampitatma
paravareso mahad-amsa-yukto
 hy ajo ’pi jato bhagavan yathagnih
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
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 by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
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. Karmis, jñanis and yogis all hanker to possess some material or spiritual assets. Karmis want material possessions, jñanis and yogis 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 karmis, jñanis and yogis have their particular mentalities in the modes of nature, and therefore they are called itara or nondevotees. These itaras, including even the yogis, sometimes harass the devotees of the Lord. Durvasa Muni, a great yogi, harassed Maharaja Ambarisa because the latter was a great devotee of the Lord. And the great karmi and jñani Hiranyakasipu even harassed his own Vaisnava son, Prahlada Maharaja. 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 Krsna 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 Narayana,” and others said, “He is the incarnation of Ksirodakasayi Visnu.” But actually He is the original Supreme Personality of Godhead — krsnas tu bhagavan svayam — and Narayana, the purusas and all other incarnations accompany Him to function as different parts of His pastimes. Mahad-amsa-yuktah indicates that He is accompanied by the purusas, who create the mahat-tanva. It is confirmed in the Vedic hymns, mahantam vibhum atmanam.
Lord Krsna appeared, just like electricity, when there was friction between Kamsa and Vasudeva and Ugrasena. Vasudeva and Ugrasena were the Lord’s devotees, and Kamsa, a representative of the karmis and jñanis, was a nondevotee. Krsna, as He is, is compared to the sun. He first appeared from the ocean of the womb of Devaki, and gradually He satisfied the inhabitants of the places surrounding Mathura, just as the sun enlivens the lotus flower in the morning. After gradually rising to the meridian of Dvaraka, the Lord set like the sun, placing everything in darkness, as described by Uddhava.
Srimad Bhagavatam Canto 03, Chapter 02, Text 14
Srimad Bhagavatam Canto 03, Chapter 02, Text 16