Among all the living entities wandering throughout the universe, one who is most fortunate comes in contact with a representative of the Supreme Personality of Godhead and thus gets the opportunity to execute devotional service. Those who are sincerely seeking the favor of Krishna come in contact with a guru, a bona fide representative of Krishna. The Mayavadis indulging in mental speculation and the karmis desiring the results of their actions cannot become gurus. A guru must be a direct representative of Krishna who distributes the instructions of Krishna without any change. Thus only the most fortunate persons come in contact with the guru. As confirmed in the Vedic literatures, tad-vijnanartham sa gurum evabhigacchet: one has to search out a guru to understand the affairs of the spiritual world. Srimad-Bhagavatam also confirms this point. Tasmad gurum prapadyeta jijnasuh sreya uttamam: one who is very interested in understanding the activities in the spiritual world must search out a guru–a bona fide representative of Krishna.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Fifth Canto, Chapter 17 – Text 11