Unless you've been raising your kids on another planet, you've probably heard of Robert Munch's classic children's book Love You Forever.  

In fact, you're probably sick of the story after reading it over and over to your own kids. But just in case you need a reminder, it's about a mother who sings the same song to her son at different stages in life. At the end of the book, the boy (now a grownup) holds his elderly mother and sings back the same song.  

Parents everywhere fell in love with the classic story, illustrator Sheila McGraw's art, and the beautiful message that you will always love your kids, even if they get on your nerves sometimes.

Since 1986, millions of copies of the book have been sold, and it reached number four on Publisher Weekly's list of best-selling children's books. But there's a tragic story behind the book that most parents know about.

It was to do with the song repeated throughout the book:

I'll love you forever, I'll like you for always, as long as I'm living my baby you'll be.

It turns out that song had a special meaning for Munsch and his wife, at a time in their lives when the author was so sad he couldn't even bring himself to cry.

The children's book author revealed to HuffPost that his wife had two stillbirths back to back.

After the second time, doctors gave the couple the heartbreaking news that they would probably never conceive a child. It was awful for both of them, but especially ironic for Munsch, who has a master's degree in Child Studies and writes books that parents share with their own children.

"You know when someone walks up to you and gives you a sucker punch right in the solar plexus?" Munsch said about hearing the news, "[It felt] like that."

Munsch came up with the "I'll Love You Forever" song as his way of crying, and would never say it aloud or write it down, he simply hummed the tune.

Then one day the story behind the book came to Munsch all at once while he was performing some of his stories for an audience. Munsch read a version of the book to them right away, and parents were in tears by the time he finished.

While publishers told Munsch the story was "too dark" to be a children's story, it has since become the most popular of his more than 50 books.

Today, Munsch and his wife are the adoptive parents of three children, and he says the tragic story behind the song isn't his anymore.

"For someone who picks up the book, it's their story, not mine."

Share this story if you remember reading Love You Forever.

[H/T: HuffPost]