Never had favourites, quality street or roses? Maybe these are only in the UK? You know what you could get, but not how many of each type.

Yes, though I get the feeling that being uncertain about the precise quantum of caramels versus mint cremes isn't exactly the point Forrest Gump was trying to make.

(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.)

It's from the US. Sees candy boxes are loaded with random chocolates and you won't know what the chocolates contain until you put one in your mouth.

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.

https://www.youtube.com/watch?v=CJh59vZ8ccc

Yes, I saw Forrest Gump when it was in cinemas. I do understand the theoretical concept of a boutique, hand-selected tray of chocolates which one cannot guess until after they're opened. It's just not something I've personally experienced.