linux + why I cant change the passwd

I have redhat 6.x version

I am trying to change the passwd from single user mode but I get the following

passwd root
changing password for user root
passwd: permission denied 

then I try to verify filesystem is mounted in read/write

 mount -o remount,rw /

but I get permission denied again when typing passswd root

please help me to understand why I get permission denied from single user mode?