In the Sivananda Lahari, Sri Sankaracharya explained the nature of Bhakti in the beautiful verse:
Ankolam Nijabijasantatir-ayaskantopalam Suchika,
Sadhvi Naijavibhum Lata Kshitiruham Sindhuh Saridvallabham;
Prapnotiha Yatha Tatha Pasupateh Padaravindadvayam,
Chetovrittirupetya Tishthati Sada Sa Bhaktirityuchyate
Here the Bhakta's intense love for God is compared to five well-known examples.
The Ankola fruits fall to the ground and the seeds are liberated. But they are instantly and powerfully attracted to the trunk of the mother tree. They move in the direction of the trunk and stick to it.
The needle flies to the magnet, attracted by an irresistible force.
A devout and chaste wife lives constantly in the thought of her husband and his service.
A creeper restlessly searches for a tree to entwine itself; and once it has caught a tree it winds itself round it inextricably as it were, with great affection and love. Even if the creeper is violently pulled away from the tree, the instant it is released it will rush back to the tree and wind itself around it.
The river bouncing towards the ocean surmounts all obstacles and flows ceaselessly till it attains the ocean.
Such should be the devotion of the Bhakta to the Lordconstant, intense and powerful.
Sravanam kirtanam vishnoh Smaranam padasevanam;
Archanam vandnam dasyam Sakhyam-atmanivedanam.
Sri Vishnu-Sravane Parikshidabhavad-Vaiyasikah Kirtane,
Prahladah Smarane Tadanghribhajane Lakshmih Prithuh Pujane;
Akrurastvabhivadane-tha Hanuman Dasye-tha Sakhyerjunah,
Sarvasvatmanivedane Balirabhut Kaivalyamesham Padam.
Hearing the Lord's stories, singing His glories, remembering Him always, serving at His feet, worshipping Him, adoring Him, being His humble servant, being His friend and dedicating one's all to Him, these are the nine ways of attaining Him.
Parikshit attained Him by hearing His stories; Vyasa's son by singing His glories; Prahlada by constantly remembering Him. Lakshmi by serving at His feet; Prithu by worshipping Him; Akrura by adoring Him; Hanuman by being His humble servant; and Bali by dedicating his all to Him. All attained salvation equally, though by different methods.