Maybe this is a generational thing, but the idea of not knowing what you’ll get in a box of chocolate is an entirely foreign notion to me.
(And though I have absolutely no idea, one might suspect that modern boxes of supermarket chocolate are assembled by machine and all contain a constant, predetermined number of each type.)
The quote was made famous by an old US movie starring Tom Hanks. The movie is called Forrest Gump. Fittingly, the character who said that quote is mentally challenged.