Why is Roger unable to say what he wants to say to Mrs. Jones at the end of the story in "Thank you, M’am" by Langston Hughes?

Roger is overcome with emotion at the end of “Thank you, M’am” by Langston Hughes, which leaves him unable to say more than a simple “thank you.”
Roger is a young man who is the product of his Harlem environment. There is no evidence of family support in his young life. When he attempts to snatch the purse off the shoulder of Mrs. Luella Bates Washington Jones, he is in for the lesson of a lifetime.
 Mrs. Jones does not report him to authorities, but instead takes him to her rooming house, where she shows him kindness and understanding. She has him wash up before they eat a meager dinner together. More importantly, she respects his circumstances and shares some of her background with him. While he is in her company, she allows him to learn how to be trustworthy.
When it is time for her to rest, she hands him the money he needs to buy the blue suede shoes that drove him to steal in the first place. Roger is unaccustomed to this type of treatment and he finds it so overwhelming that he is virtually speechless. Deep within, Roger realizes Mrs. Jones gave him much more than the money for those shoes.