Based on Chapter 20, why is a Guru's grace considered essential for understanding profound spiritual texts like the Upanishads?

Chapter 20 strongly emphasizes the necessity of a Guru for true spiritual understanding. It explains that the Upanishads contain the essence of the Vedas and are very difficult to grasp without the grace of Hari (God) and the Guru. The chapter states that a scholar who arrogantly believes they can comprehend these texts through their own efforts will not succeed, as the hidden truth is a difficult path at every step without a guide. Conversely, for a humble devotee attached to the Guru's feet, the hidden meaning opens up automatically and without difficulty, as this is the science of Self-knowledge.