What caused Dev to feel 'shame' and 'dejection' after learning the truth about the Sanyasi?

Dev was overcome with shame and dejection upon learning the truth because he realized he had unjustly doubted and blamed his Guru, Sai Baba. When Jog's letter revealed that the Sanyasi was Baba in disguise, Dev was struck by the greatness of Baba's actions and the foolishness of his own lack of perception. The text in Chapter 40 states he exclaimed, 'Shame, shame! I blamed Sai in vain!' and felt dejected. His shame was rooted in his 'pride of knowledge' and his failure to see past the physical form, causing him to miss the profound, loving way Baba had kept his word.