I think that there is a lesson in the character of the postmaster. Tagore takes his character and constructs it with every bit of the rationality, intelligence, and sensibility of an educated fellow in India. The postmaster is obviously literate, capable of profound thought, and is able to recognize his own state of being in the world. He is not one who slaves for wages and is unaware of the world and his place in it. He longs to leave the village and return to the urbane setting of Calcutta. Yet, in this characterization, Tagore is deliberate enough to make a character that lacks an emotional affect. The postmaster is kind of whiny and frail, in that he becomes dependent on Ratan, the orphan, to fill the void that his mother/ sister fulfilled for him in terms of taking care of him. Whereas Ratan looks at him with nothing but emotional sensibilities, the postmaster looks at her with more of a cerebral approach, than anything else. It is why the pinnacle of the story makes so much sense. When he laughs and dismisses her plea to go with him back to Calcutta, he does so from an intellectual standpoint. Logically, it would make no sense for her to accompany him. Tagore does this to bring out the fact that while the postmaster might be intelligent and "book smart," he lacks the interpersonal knowledge to see that such a reaction is crushing to the orphan Ratan, for whom association and belonging have not been experienced on a consistent basis in her own life. This is repeated when their last moment consists of him offering money for her services, while she denies it and runs away. Again, an emotional response is blighted by an intellectual or cerebral one. We see this philosophical bent reveal itself in the end of the story when the postmaster leaves and wonders about the pain he caused upon her, something he is able to dismiss because of his own philosophical justifications or explanations. Tagore undercuts this in the last paragraph of the story in saying that Ratan did not have such a background to help her cope with her own pain, suffering that could only be felt by an orphan "weeping copiously."
The lesson here, if there is one, is that there cannot be any real justification behind the breaking of another person's heart. Emotional cruelty is both unavoidable, and yet inexcusable. The only hope one can have is possessing enough emotional intelligence to be aware of it and to be mindful of another person's pain. This is something that the postmaster either is not or chooses not to be. In the end, this has to be the lesson present in his character, one that Tagore seems to enjoy building up and then taking apart through the same high powered, laser analysis of thought and rationality that is such a part of the postmaster's character.