Aram speaks to being surprised in going horse riding with his cousin because the reality surpassed in so many ways the reality in which Aram and his tribe lived. The initial moment of seeing Mourad and the horse outside his window in the early morning was a surprising one:
In the first place, my earliest memories had been memories of horses and my first longings had been longings to ride.
This was the wonderful part.
In the second place, we were poor.
This was the part that wouldn't permit me to believe what I saw.
One reason why riding the horse was such a surprise was the result of this cognitive dissonance between what reality presents and what one's mind knows to be true. Aram knows that his family is poor and yet the spectacle the beautiful white horse and the opportunity to ride it is a reality that would not "permit" him to believe what he saw.
Another reason why he is surprised is because he knows that Mourad stole the horse. He knew that Mourad stole the horse and Aram knows that "No member of the Garoghlanian family could be a thief." The tribal connection and faith that Aram has is one that prevents him from embracing the idea of theft. Seeing the horse in front of his window and his cousin riding on it leads him to recognize that Mourad broke convention in stealing the horse. As a result, Aram is surprised at the reality that is in front of him and the process it took to cause that reality to materialize. This chasm between reality and the events that precipitate it are another source of his surprise in the story.