svayaṁ phala-rūpateti brahma-kumārāḥ.
स्वयं - by itself (without any other cause) फलरूपता - give its reward; इति - thus; ब्रह्मकुमाराः - the sons of Brahmā
Bhakti is its own fruit, thus say the sons of Brahmā.
The view that "bhakti is its own fruit" is held by the sons of Brahmā. We need to remember that Narad Ji is also a son of Brahma and so take this as Narad Ji’s views as well.
Devotion to God does not depend on anything else besides love itself. Love begets love and the sole purpose of divine love is just love. The more one expresses their love for God, the greater their love grows, and the ultimate aim of the devotee is simply to love. The means of reaching this goal is also through one's love for God.
Here are some quotes from the Bhagavata Purana (Srimad Bhagavatam), which support the view that love for God is the means and the end of devotion.
SB. 3.5.25
सतां प्रसङ्गान्मम वीर्यसंविदो
भवन्ति हृत्कर्णरसायना: कथा: ।
तज्जोषणादाश्वपवर्गवर्त्मनि
श्रद्धा रतिर्भक्तिरनुक्रमिष्यति ॥ २५ ॥
satāṁ prasaṅgān mama vīrya-saṁvido
bhavanti hṛt-karṇa-rasāyanāḥ kathāḥ
taj-joṣaṇād āśv apavarga-vartmani
śraddhā ratir bhaktir anukramiṣyati
When one associates with pure devotees and engages in discussions about the pastimes and activities of the Supreme Personality of Godhead, it is a deeply satisfying experience for the heart and the mind. Through such spiritual engagement, one can gradually advance on the path of liberation, and become free from the material world. As one's attraction to the Lord becomes stronger, they develop a fixed determination for devotion, and begin to engage in genuine devotional service.
ŚB 5.18.12
यस्यास्ति भक्तिर्भगवत्यकिञ्चना
सर्वैर्गुणैस्तत्र समासते सुरा: ।
हरावभक्तस्य कुतो महद्गुणा
मनोरथेनासति धावतो बहि: ॥ १२ ॥
yasyāsti bhaktir bhagavaty akiñcanā
sarvair guṇais tatra samāsate surāḥ
harāv abhaktasya kuto mahad-guṇā
manorathenāsati dhāvato bahiḥ
When someone has unalloyed devotion for God, they acquire the same good qualities as the demigods, such as religion, knowledge, and renunciation. On the other hand, a person who lacks devotion and is caught up in materialistic pursuits has no good qualities, even if they are skilled in practices like mystic yoga or taking care of their family. Such a person is driven by their own ideas and is bound to serve the Lord's external energy. Therefore, how could they possess any good qualities?
ŚB 11.2.42
भक्ति: परेशानुभवो विरक्तिरन्यत्र चैष त्रिक एककाल: ।
प्रपद्यमानस्य यथाश्नत: स्युस्तुष्टि: पुष्टि: क्षुदपायोऽनुघासम् ॥ ४२ ॥
bhaktiḥ pareśānubhavo viraktir
anyatra caiṣa trika eka-kālaḥ
prapadyamānasya yathāśnataḥ syus
tuṣṭiḥ puṣṭiḥ kṣud-apāyo ’nu-ghāsam
If someone takes shelter of the Supreme Personality of Godhead, they experience three things at the same time: devotion, a direct experience of the Lord, and detachment from everything else. This is similar to how someone feels pleasure, nourishment, and relief from hunger when they eat, and with each bite, these feelings increase.