In the story "The Jilting of Granny Weatherall," why is Granny concerned about the box in the attic?

Let us remember that this incredible story allows us to
see the dying thoughts and cares and concerns of a woman who is thinking about preparing
herself for death. She has so many things that she wants to sort out before dying, and
one of these things is the box of letters in the attic that your question refers to. Let
us note what the text tells us about this box of
letters:

The
box in the attic with all those letters tied up, well, she'd have to go through that
tomorrow. All those letters--George's letters and John's letters and her letters to them
both--lying around for the children to find afterward made her uneasy. Yes, that would
be tomorrow's business. No use to let them know how silly she had been
once.

Thus we can see that
the letters in the box are from her former lovers, George and John. Granny is thinking
about them because she doesn't want her children to ever see those letters and see a
different, more romantic side from the stern, indominatable mother who shows herself in
this story. It is interesting that what dominates this tale is the lingering hurt that
George caused Granny Weatherall when he jilted her. Her reluctance for anyone else to
see the letters from this time likewise indicates the way that she tries to keep this
deep wound secret.