The difference between vaidhi and raganuga sadhana-bhakti is the inspiration that drives them. In vaidhi bhakti the inspiration comes from the sense of obligation one feels to follow the instructions of the spiritual master and the scripture. That obligation may be based on fear of what will happen if one does not follow, one’s sense of duty, which is usually based on scriptural injunction, or habit. The practice of raganuga bhakti however, is inspired by one’s natural attraction to the devotional service itself. Therefore, it is important for us to develop and then maintain our inspiration to serve the Lord. It helps if we understand first why we are obliged to perform devotional service at all.
(LFD, Vol2, Purpose of Sädhana-bhakti, Pg 15, 2nd & 3rd paragraph)