Devotional service means purifying the senses. In the conditioned state our senses are covered by material sense gratification, and as long as one is not trained in purifying the senses, one cannot become a devotee
SB Canto 7
My dear Lord, O infallible one, my position is like that of a person who has many wives, all trying to attract him in their own way. For example, the tongue is attracted to palatable dishes, the genitals to sex with an attractive woman, and the sense of touch to contact with soft things. The belly, although filled, still wants to eat more, and the ear, not attempting to hear about You, is generally attracted to cinema songs. The sense of smell is attracted to yet another side, the restless eyes are attracted to scenes of sense gratification, and the active senses are attracted elsewhere. In this way I am certainly embarrassed.
The human form of life is meant for God realization, but this process, which begins with śravaṇaṁ kīrtanaṁ viṣṇoḥ (SB 7.5.23)—hearing and chanting of the holy name of the Lord—is disturbed as long as our senses are materially attracted. Therefore devotional service means purifying the senses. In the conditioned state our senses are covered by material sense gratification, and as long as one is not trained in purifying the senses, one cannot become a devotee. In our Kṛṣṇa consciousness movement, therefore, we advise from the very beginning that one restrict the activities of the senses, especially the tongue, which is described by Śrīla Bhaktivinoda Ṭhākura as most greedy and unconquerable. To stop this attraction of the tongue, one is authoritatively advised not to accept meat or similar uneatable things nor to allow the tongue to hanker to drink or smoke. Even the drinking of tea and coffee is not permitted. Similarly, the genitals must be restricted from illicit sex. Without such restraint of the senses, one cannot make advancement in Kṛṣṇa consciousness. The only method of controlling the senses is to chant and hear the holy name of the Lord; otherwise, one will always be disturbed, as a householder with more than one wife would be disturbed by them for sense gratification.