When transcendental devotional service, by which love for Krishna is attained is executed by the senses, it is called Saadhana Bhakti, or the regulative discharge of devotional service. Such devotion eternally exists within the heart of every living entity. The awakening of this eternal devotion is the potentiality of devotional service in practice. — Sri Caitanya Caritamrta, Madhya Lila, 22.105.
Devotional Service is just like a seed sown in the heart of a living entity and if he goes on hearing & chanting, that seed fructifies, just as the seed of a tree fructifies with regular watering. Thus the spirtitual plant of devotional Service grows gradually.
Although karma, jnana and yoga are discussed in Bhagavad-gita, the only purpose is to inform us of Krishna’s greatness and engage us in His devotional service.
Lecture on "Understanding Krishna through Bhagavad Gita" – Salem – 8th Nov'06
Loving devotional service to the Lord begins with hearing about the Lord.
Srimad Bhagavatam 1.7.7,
When we expose ourselves to severe vaisnava-aparadha, it is like pulling our devotional creeper out from the root. It doesn't get the nourishment then we systematically dry out, then we lose the taste for chanting, reading, deity worship, book distribution, what could be worse! We lose devotional service ultimately.
Devotional service is so simple, but devotees because of their material inclinations make it complicated.