I can honestly say I have never heard that saying, but I think I understand what your Grandfather was saying and he was a wise man.
Your liver holds emotions, (they have to be somewhere in your body, you know) so as you flush away the stones and gunk you have had in your liver and gallbladder, it just makes sense you will have a sense of well being that you never had before! "The peace that passes all understanding," as the Bible says.
You know, "shrinks" should tell their patients to flush instead of taking all the mind changing drugs they prescribe to "help" their patients!