What is the narrator's reaction to Mrs. Croft's goodbye? What does this reveal about his development?

The narrator was a bit disappointed the way Mrs. Croft coldly bid goodbye to him.

This reveals that the narrator was attached to Mrs. Croft and this is why her cold goodbye disappointed him.

Explanation:

"Interpreter of Maladies" is a collection of short stories written by Jhumpa Lahiri. The last story in the collection titled "The Third and the Final Continent" is a story of an Indian man who left his country to study and work in North America.

The narrator shifted to Mrs. Croft house on rent and spend six weeks there before moving out.  In these six weeks, he became affectionate towards her, especially after knowing her age– 103 years. So, when Mrs. Croft simply said "Goodbye, then!" and shut the door behind the narrator, without saying anything else, he was disappointed.

He got disappointed with her goodbye because he has become attached to her and expected some warm goodbye.