Bhaktivinoda Thakura explains that sometimes when one realizes the futility of material existence, because of prolonged endeavors to fulfill his desires, he realizes the futility of materialistic life. After prolonged endeavor to fulfill his desires, he either does not get them fulfilled or they do get fulfilled, but he realizes that that fulfillment of the desires cannot satisfy him. At that point, some detachment is awakened within his heart, and then by good fortune, if he gets an opportunity to associate with a devotee of Krishna and he hears from Krishna that the highest goal is to attain devotional service and the lotus feet of Krishna,
then he gains some faith in devotional service.
LFD Vol 3, Establishing Our relationship with Lord, Pg 67, 4th paragraph.