It seems counterintuitive that Sai Baba would choose such a cynical and argumentative person to be his biographer. Why was he selected?

The text suggests that Sai Baba's choice was an act of compassion and a means of transformation for the author. Although Chapter 2 describes the author's initial flaws—being "mischievous, talkative, cynical, and critical"—it also notes that Sai felt compassion for him and wished to calm his "impatient mind." Baba's instruction to write was coupled with a critical condition for receiving His full help: the author had to dissolve his ego and surrender completely. This implies the act of writing the stories was the very process through which the cynical author would be transformed into a devoted scribe, making him a living example of Baba's power to change a person's nature.