Philosophically it is based on the Sanskrit texts Bhagavad-gītā and the Bhagavat Purana, or Srimad Bhagavatam. These are the historic texts of the devotional bhakti yoga tradition, which focuses on the spiritual nourishment of the soul thereby guaranteeing inner happiness and contentment for the sincere practitioner.
The basic philosophy begins with the understanding that we are not this body, rather we are the conscious spiritual beings residing in these varieties of material bodies resembling humans, aquatics, plants, etc.
“Those who are beyond the dualities that arise from doubts, whose minds are engaged within, who are always busy working for the welfare of all living beings, and who are free from all sins achieve liberation in the Supreme.” ~ Bhagavad Gita 5.25 (Give a Blue Background with white text)
Connection of the soul with the supreme through loving devotional service is called Bhakti Yoga.
Real happiness can be derived only from a spiritual source, just like a fish can derive happiness only in water, not outside. We are spirit souls and our happiness comes from loving relationships with other spiritual beings, what to speak of the joy that is experienced when we are connected to the supreme spiritual being, Krishna.
Thus to love and be loved is real spiritual nature.
” O King, constant chanting of the holy name of the Lord after the ways of the great authorities is the doubtless and fearless way of success for all, including those who are free from all material desires, those who are desirous of all material enjoyment, and also those who are self-satisfied by dint of transcendental knowledge.”
~ Srimad Bhagavatam 2.1.11