Dear Ms. Bonacci,

Thank you for this information. Since writing, I now have evidence
that the saying is very unlikely to be original with Bergman.

John Logan, a reference librarian here at Princeton, pointed out to me
that when several very different sources for a quotation are given (in
this case Bergman, Dale Carnegie, and others), it is likely that the
real source is unknown. He referred me to A New Dictionary of
Quotations on Historical Principles from Ancient and Modern Sources,
ed. H.L. Mencken (New York: Knopf, 1942), which lists the saying as
author unidentified (p. 512).

In 1942 Ms. Bergman was 27 years old and had just made Casablanca. It
seems a bit early for her to have coined a quote on happiness.
However, the quote still seems consistent with her attitude several
decades later in the Fallaci interview in The Egotists (also published
in Look). As you indicate, the remaining possibilities are that she
said it in a non-English-language publication or in a televised interview.

Thanks to all!

All the best,
Sam Wang