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CC Ādi 4.15-16

prema-rasa-niryāsa karite āsvādana
rāga-mārga bhakti loke karite pracāraṇa
rasika-śekhara kṛṣṇa parama-karuṇa
ei dui hetu haite icchāra udgama
Synonyms: 
prema-rasa — of the mellow of love of God; niryāsa — the essence; karite — to do; āsvādana — tasting; rāga-mārga — the path of spontaneous attraction; bhakti — devotional service; loke — in the world; karite — to do; pracāraṇa — propagation; rasika-śekhara — the supremely jubilant; kṛṣṇa — Lord Kṛṣṇa; parama-karuṇa — the most merciful; ei — these; dui — two; hetu — reasons; haite — from; icchāra — of desire; udgama — the birth.
Translation: 
The Lord’s desire to appear was born from two reasons: the Lord wanted to taste the sweet essence of the mellows of love of God, and He wanted to propagate devotional service in the world on the platform of spontaneous attraction. Thus He is known as supremely jubilant and as the most merciful of all.
Purport: 

During the period of Lord Kṛṣṇa’s appearance, the killing of asuras or nonbelievers such as Kaṁsa and Jarāsandha was done by Viṣṇu, who was within the person of Śrī Kṛṣṇa. Such apparent killing by Lord Śrī Kṛṣṇa took place as a matter of course and was an incidental activity for Him. But the real purpose of Lord Kṛṣṇa’s appearance was to stage a dramatic performance of His transcendental pastimes at Vrajabhūmi, thus exhibiting the highest limit of transcendental mellow in the exchanges of reciprocal love between the living entity and the Supreme Lord. These reciprocal exchanges of mellows are called rāga-bhakti, or devotional service to the Lord in transcendental rapture. Lord Śrī Kṛṣṇa wants to make known to all the conditioned souls that He is more attracted by rāga-bhakti than vidhi-bhakti, or devotional service under scheduled regulations. It is said in the Vedas (Taittirīya Up. 2.7), raso vai saḥ: the Absolute Truth is the reservoir for all kinds of reciprocal exchanges of loving sentiments. He is also causelessly merciful, and He wants to bestow upon us this privilege of rāga-bhakti. Thus He appeared by His own internal energy. He was not forced to appear by any extraneous force.