I always assumed it came from this:
"And acceptance is the answer to all my problems today. When I am disturbed, it is because I find some person, place, thing or situation - some fact of my life - unacceptable to me, and I can find no serenity until I accept that person, place, thing or situation as being exactly the way it is supposed to be at this moment."
I suppose it isn't saying it's impossible to change those things, but rather that we should accept them. Some are, of course, impossible to change.
(BB 1st ed.)