Why did the narrator want to return the letter in The Best Christmas Present in the World?

In the story by Michael Morpurgo the narrator buys a desk at a junk shop and discovers a hidden tin box with a letter dated December 26 1914 from Captain Jim Macpherson to his wife Connie.

The narrator felt a moral responsibility to deliver the letter because it represented a last communication from Jim to Connie; she had preserved it all these years, reading it repeatedly to feel his presence. Returning it was a way of honouring that memory and giving closure.

Furthermore, when the narrator visits Mrs Macpherson in her nursing home to return the letter and the box he found, she mistakes him for her husband Jim because she is old and suffering memory loss. In that instant she believes Jim has returned on Christmas as he promised in his letter. The narrator realises that for her, his visit becomes “the best Christmas present in the world”  not because of the physical letter alone, but because her hope and delusion are fulfilled in that moment.

So the narrator wanted to return the letter out of respect for the attachment Mrs Macpherson had to it, out of a desire to complete the story and give her that moment of fulfilment.