Why is a Guru considered so necessary for spiritual progress according to Chapter 50?

Chapter 50 stresses the indispensable role of a Guru in overcoming ignorance, which is said to manifest from doubt (vikalpa). The text warns that when doubt contaminates knowledge, even a very wise person can become prideful, citing the great knower Shuka who suffered a loss due to doubt. This corruption of knowledge is compared to how a single drop of fermented gruel can spoil a large pot of milk. Therefore, the text concludes that ignorance cannot be removed without a Guru. A Guru's guidance is essential to first understand and then eliminate this doubt, which in turn purifies the mind and allows for the manifestation of infinite knowledge and non-dual Samadhi.