The Supreme Personality of Godhead is self-sufficient, but to enjoy His transcendental bliss He requires the cooperation of His devotees. In Vrindavana, for example, although Lord Krishna is full in Himself, He wants the cooperation of His devotees like the cowherd boys and the gopis to increase His transcendental bliss. Such pure devotees, who can increase the pleasure potency of the Supreme personality of Godhead, are certainly most dear to Him. Not only does the Supreme Personality of Godhead enjoy the company of His devotees, but because He is unlimited He wants to increase His devotees unlimitedly. Thus, He descends to the material world to induce the nondevotees and rebellious living entities to return home, back to Godhead. He requests them to surrender unto Him because, unlimited as He is, He wants to increase His devotees unlimitedly. The Krishna consciousness movement is an attempt to increase the number of pure devotees of the Supreme Lord more and more. It is certain that a devotee who helps in this endeavor to satisfy the Supreme Personality of Godhead becomes indirectly a controller of the Supreme Lord. Although the Supreme Lord is full in six opulences, He does not feel transcendental bliss without His devotees. An example that may be cited in this regard is that if a very rich man does not have sons in a family he does not feel happiness. Indeed, sometimes a rich man adopts a son to complete his happiness. The science of transcendental bliss is known to the pure devotee. Therefore the pure devotee is always engaged in increasing the transcendental happiness of the Lord.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Ninth Canto, Chapter 4 – Text 64