CC Ādi 17.275
catur-vidha bhakta-bhāva kare āsvādana
Servitude, friendship, parental affection and conjugal love for the Supreme Personality of Godhead are the basis of the four kinds of devotional activities. In śānta, the marginal stage of devotional service, there is no activity. But above the śānta humor are servitude, friendship, parental affection and conjugal love, which represent the gradual growth of devotional service to higher and higher platforms.