…The appearance of Kṛṣṇa is the answer to all imaginative iconography of the Supreme Personality of Godhead. Everyone imagines the form of the Supreme Personality of Godhead according to his mode of material nature. In the Brahmā-saṁhitā it is said that the Lord is the oldest person. Therefore a section of religionists imagine that God must be very old, and therefore they depict a form of the Lord like a very old man. But in the same Brahmā-saṁhitā, that is contradicted; although He is the oldest of all living entities, He has His eternal form as a fresh youth.
…”O dear Lord,” the demigods continued, “You are unborn; therefore we do not find any reason for Your appearance other than for Your pleasurable pastimes.” Although the reason for the appearance of the Lord is stated in the Bhagavad-gītā (He descends just to give protection to the devotee and vanquish the nondevotee), actually He descends for His pleasure-meeting with the devotees, not really to vanquish the nondevotees. The nondevotees can be vanquished simply by material nature. “The action and reaction of the external enregy of material nature (creation, maintenance and annihilation) are being carried on automatically. But simply by taking shelter of Your holy name–because Your holy name and Your personality are nondifferent–the devotees are sufficiently protected.” The protection of the devotees and the annihilation of the nondevotees are actually not the business of the Supreme Personality of Godhead when He descends. They are just for His transcendental pleasure. There cannot be any other reason for His appearance.
Excerpts from; KRSNA, The Supreme Personality of Godhead
By His Divine Grace A.C. Bhaktivedanta Swami Prabhupada
Volume 1, Chapter 2
The Hare Krishna Movement 