Can't permanently change Auto Empty Delete folder option - why?

Every time I open Outlook, I must manually change the Option to allow me to
automatically empty the Deleted Items folder upon closing Outlook. I go into
"Options" and check the box that is supposed to automatically empty the
folder. When I close Outlook, it does empty the folder, but then, when I
open Outlook again, the box has reverted back to being unchecked. This
defeats the purpose of my checking the box in the first place. Why won't it
stay checked?