“One who is actually intelligent, although he may be a devotee free from material desires, a karmi desiring all kinds of material facilities, or a jnani desiring liberation, should seriously engage in bhakti-yoga for the satisfaction of the Supreme Personality of Godhead.” SB 2.3.10
“Devotional service to Krishna is the chief function of the living entity. There are different methods for the liberation of the conditioned soul—karma, jnana, yoga and bhakti—but all the others are dependent on bhakti.
“The other processes cannot yield results unless they are associated with devotional service. Devotional service, however, is so strong and independent that it can give one all the desired results.” CC Madhya 24.92
“With the exception of devotional service, all the methods of self-realization are like the nipples on the neck of a goat. Therefore an intelligent person adopts only devotional service, giving up all other processes of self-self-realization.”
Without devotional service, other methods for self-realization and spiritual life are useless. Other methods cannot produce good results at any time, and therefore they are compared to the nipples on the neck of a goat. These nipples cannot produce milk, although it may appear that they can. An unintelligent person cannot understand that only devotional service can elevate one to the transcendental position.