Emacs find-file default-directory

After upgrade Mac OS X from Lion to Mavericks, emacs find-file default directory was changed too. Before upgrade, find-file default directory is $HOME. After upgrade, find-file default become "/". I have no idea about the reason, but adding this line to .emacs.d/init.el makes this problem resolved.

(setq default-directory "/Users/$USER_NAME/")

$USER_NAME has to be changed each environment.