Box of Chocolates is a container item purchased for 10 from a Lovely Merchant in any capital city during the Love is in the Air seasonal event. While it does not have a duration listed in the tooltip, it may vanish at the end of the event.
This item will contain six of the following, chosen at random:
You must consume one of each of the four chocolates to complete the achievement. Since they bind on pickup, you must obtain your own Box(es) of Chocolates. It typically takes two Boxes of Chocolates to complete this achievement, but getting all four chocolates in one box is by no means rare, and it can require several Boxes of Chocolates to get that last one.
Sweet Tooth is in turn required for the Fool For Love World Event meta-achievement.
See the talk page for a discussion of the odds of each item.