The Lord instructed Śrīla Rūpa Gosvāmī to write the transcendental literature named Bhakti-rasāmṛta-sindhu, the science of devotional service, and indicate therein the substance of the five transcendental relationships with the Lord—śānta-, dāsya-, sakhya-, vātsalya- and madhura-rasa. It is explained in that great literature how the transcendental relationship of śānta-rasa, taking the shape of unflinching faith in Kṛṣṇa, is further developed into dāsya-rasa with the spirit of service, and then to sakhya-rasa, or undeterred fraternity, and further to the transcendental platform of parental love, wherein one feels himself to be maintaining the Lord. All these transcendental relationships culminate on the highest platform, conjugal love (madhura-rasa), wherein they all exist simultaneously.
TLC 2: Sanātana Gosvāmī