Chapter Six of The Princess Bride is a crucial one in the presentation of the theme of life not being fair.
When young Bill's father is reading to him, the boy is sure his dad misspoke when he read the line about how Buttercup and Prince Humperdinck's wedding took place "between the ministers' meeting and the treasury whatever." His father tells him he read the exact words on the page and didn't skip anything, and then stops reading for the night. Bill recalls that he was "rocked" by the idea that the heroine of a story could marry the villain and not the hero. It just couldn't happen that way.
The next evening his father continues reading, and Bill finds out the wedding was just part of a nightmare Buttercup was having. However, he still wasn't happy. Instead, "there was that damn discontent, shaking it's dark head."
This discontent stays with him until he is in his teens and becomes friends with a writer, Edith Neisser, who tells him life isn't fair. Furthermore, she says, parents are telling children a cruel lie when they say it is. Instead of being devastated, Bill is relieved to have that unspoken truth finally out in the open. "And that's what I think this book's about," he says.
In a note just for the children reading the book for the first time, adult Bill tells them the story his father read to him does have a happy ending, but that terrible, unfair things will happen before the tale ends. He says he wants the young readers to know life isn't fair so they will be prepared for the bad things to come in the story and in life.