Based on the text, what is the underlying spiritual principle demonstrated by Balasaheb Deo's difficulty and his subsequent vow?

The story of Balasaheb Deo illustrates the spiritual principle that true devotion often involves surrender when self-effort fails. Chapter 41 states that Sai is loving toward devotees and wards off their difficulties. In Deo's case, the 'difficulty' was his inability to perform a desired spiritual practice. His persistent failure led him to a deeper spiritual state: complete reliance on the Guru. By making the vow, "Only when Sai gives the order, Then only will I read the Dnyaneshwari," he moved from self-reliance to Guru-reliance. This act of setting aside his own will in favor of waiting for the Guru's grace is presented as a method by which Sai deepens a devotee's faith and adds to their devotion.