Our enthusiasm is often awakened by some outside impetus. Srila Prabhupada said, “Purity is the force.” It is not a stick that will push us, but a pure example. If we actually hear the message of Krishna consciousness presented purely, we begin to desire bhakti to grow in our own hearts. What happens when that desire begins to grow? We decide we want to act for Krishna’s pleasure. Then we simply need direction. But the actual impetus comes from the heart’s inspiration to do something for Krishna. We cannot be forced and be practicing bhakti.
Lectures from a disciple Vol 1, Pg 334 2nd paragraph.