When one’s desire to love Krishna in his particular relationship becomes intensified, this is known as pure love of Godhead. In the beginning, a devotee is engaged in the regulative principles of devotional service by the order of his spiritual master. When one thereby becomes completely purified of all material contamination, there develops an attachment and taste for devotional service. This taste and attachment, when gradually intensified in the course of time, becomes love. The word “love” can only be actually applied in a relationship with the Personality of Godhead. In the material world, love is not applicable at all. What goes on under the name of love in the material world is nothing but lust. There is a gulf of difference between love and lust, as the difference between gold and iron. In the Narada-pancaratra it is clearly stated that when lust is completely transferred to the Supreme Godhead and the concept of kinship is completely reposed in Him, such is accepted as pure love of God by great authorities like Bhisma, Prahlada, Uddhava and Narada. A great authority like Bhisma has explained that love of Godhead means completely giving up all so-called love for any other person. According to Bhisma, love means to repose one’s affection completely upon one person, withdrawing all affinities for any other person. This pure love can be transferred to the Supreme Personality of Godhead under two conditions – out of ecstasy and out of the causeless mercy of the Supreme Personality of Godhead Himself.
Source: A.C. Bhaktivedanta Swami Prabhupada (2011 edition), “The Nector of Devotion”, Page 143