Caitanya Mahāprabhu compared bhakti as the . . . a great ocean. So when He was speaking before Rūpa Gosvāmī, He said that, "It is just like a ocean. So I'll take a drop of it, and you taste it, and you'll understand what is this ocean." Just like by tasting one drop of seawater we can understand the taste of the whole ocean, similarly, Caitanya Mahāprabhu described a small portion of Bhakti-rasāmṛta-sindhu. Bhakti-rasa-amṛta. Bhakti, devotional service, there is a rasa, taste, and the taste is amṛta, eternal.
Here also, we have got taste for any relationship. Just like we have got our relationship with master and servant. So this relationship is a perverted reflection of the real master and servant. Here it is perverted, because the master also does not love the servant, and the servant also does not love the master. The servant serves the master so long there is payment. If the payment is stopped, then no more the servant will be available.
But in the eternal world, the Kṛṣṇa's servant . . . so that is eternal, without any payment. Mama janmani janmanīśvare bhavatād bhaktir ahaitukī (CC Antya 20.29, Śikṣāṣṭaka 4). Ahaitukī. The servants of God, or Kṛṣṇa, they serve Kṛṣṇa not for any material gain. Ahaitukī. Therefore this word is used, ahaitukī, "without any cause of motive."
This is real bhakti. Therefore this bhakti word is applicable only in relationship with God, or Kṛṣṇa. In the material world there cannot be any use of the word bhakti. Because here the so-called devotion, service, is motivated. So this bhakti word is monopolized by Kṛṣṇa, and nobody else.