According to the text, why is a Guru considered essential for removing doubt and achieving true knowledge?

Chapter 50 explains that a Guru is indispensable because ignorance, which manifests from doubt, cannot be removed without one. The text warns that when doubt enters knowledge, even a wise and knowledgeable person can become prideful and their wisdom becomes corrupted. It uses the powerful analogy of a single drop of fermented gruel spoiling a large pot of milk to illustrate this point. The chapter even cites the example of a supreme knower like Shuka, who suffered a loss due to doubt. Therefore, the guidance of a Guru is presented as the only way to remove doubt, purify the mind, and allow infinite, non-dual knowledge to manifest.