What causes the characters to be sad as they eat Littmus Lozenges?

In the novel Because of Winn-Dixie, feeling sad is associated with Littmus Lozenges because they have a secret ingredient. The relevant information is primarily in chapters 17 through 19, but the lozenges reappear at the very end of the book.
Opal, the protagonist, learns about the candy while in the library with Miss Franny Block (chapter 17). It was an older type of candy, first made after the Civil War and invented by Miss Franny’s great-grandfather, Littmus Block, during a period of mourning for his family members who had died of disease. She still has a lot of the candy and gives some to Opal, her friend Amanda, and the dog, Winn-Dixie. “Sorrow” is one of the ingredients, Miss Franny says, so only people who have experienced sorrow in their lives can taste that flavor. Amanda does not want to say why she was sad, and leaves the library.
In the next chapter, Opal gives a lozenge to her father, asks about Amanda, and learns that Amanda’s five-year-old brother had died, and that is why she is sad. In chapter 19, Opal gives a lozenge to Otis, who works in the pet store where Opal hangs out. Saying it reminds him of jail, he tells her a story about getting sent to jail for playing guitar and resisting arrest; part of his sadness is not being allowed to play guitar on the street any more.