How does "Kabuliwala" render the richness of the inner world of man?

The richness of the inner world of humanity is seen when Tagore recognizes that Kabuliwala's story is similar to his own.

Throughout the story, Tagore viewed Kabuliwala from a distance.  The old Afghan merchant was seen as Mini's friend, a singer of songs with the girl, a bringer of grapes and raisins, and then, as one arrested by the authorities. There was never a clear attempt to know Kabuliwala's story. 

This changes when Kabuliwala returns to visit Mini.  The recently released prisoner arrives at Tagore's doorstep on the eve of Mini's wedding.  Initially, Tagore displays a dismissive attitude towards the visitor, claiming that there are preparations for the ceremony that need to take place. However, he has a change of heart and calls Mini down to see her old friend.

The Kabuliwala is not able to process how much his little friend had changed.  After eight years, she was no longer a chatterbox girl, but a young woman about to enter the next stage of her life.  As the Kabuliwala sits on the ground, overwhelmed with the moment, Tagore recognizes the richness of his inner world: "I remembered the day when the Kabuliwala and my Mini had first met, and I felt sad."  This is the first time he recognizes the full dimensions of the Kabuliwala.  When the Kabuliwala reveals that he himself is the father of a girl about Mini's age, the intricacy and richness of his narrative is further revealed.  He communicates this not through words, but through an object:

... he put his hand inside his big loose robe, and brought out a small and dirty piece of paper. With great care he unfolded this, and smoothed it out with both hands on my table. It bore the impression of a little hand. Not a photograph. Not a drawing. The impression of an ink-smeared hand laid flat on the paper. This touch of his own little daughter had been always on his heart, as he had come year after year to Calcutta, to sell his wares in the streets.

Tagore realizes the richness of the Kabuliwala's inner world:

Tears came to my eyes. I forgot that he was a poor Kabuli fruit-seller, while I was--but no, what was I more than he? He also was a father. That impression of the hand of his little Parbati in her distant mountain home reminded me of my own little Mini.

Tagore recognizes that he bears much in common with this "Kabuli fruit-seller."  They are both fathers who are struggling with the passage of time. When Tagore gives the Kabuliwala money to go back to Afghanistan to see his daughter, it is an acknowledgement of the connection they share with one another.  Acknowledging the Kabulwala's humanity allows Tagore to feel that his daughter's wedding was "all the brighter for the thought that in a distant land a long-lost father met again with his only child."

The richness of humanity's inner world is understood when we realize that we have more in common than our differences. There is much in way of difference between Tagore, the accomplished writer, and a fruit seller from Kabul.  Yet, in one moment, Tagore recognizes the rich, inner world of the Kabuliwala, and in acknowledging it, he validates their shared narratives.