Actually bhakti should be automatic and spontaneous. There should be no motive in serving Kṛṣṇa, but even if there is a motive, service rendered unto Kṛṣṇa is good. Even if one approaches Kṛṣṇa with some ulterior motive, one is considered pious. For instance, Dhruva Maharaja initially worshiped Kṛṣṇa with a motive, but after attaining perfection in devotional service, his ulterior motive vanished. When he actually saw Kṛṣṇa, he said, “I do not want anything from You. I don’t want any benediction other than Your service.” After hearing about the many transcendental qualities of Kṛṣṇa, if we somehow or other become attracted to Kṛṣṇa consciousness, our lives will be successful. Tasmat kenapy upayena manah krsne nivesayet: “Somehow or other we have to attach our minds to Kṛṣṇa consciousness.” (BG 7.1.32) Then Kṛṣṇa will help us and give us intelligence from within.
Source: A.C. Bhaktivedanta Swami Prabhupada (2007 edition), “Teachings of Lord Kapila, The Son of Devahuti”, Page 181