One key element is the relationship between the children in real life. Just as in the book, there was a bond between the kids. Also, the unexpected death was crucial. It led to the exploration of themes like how to deal with grief and how a friendship can be both a source of joy and pain. Another element could be the environment they were in, which might have inspired the creation of Terabithia in the story.
Yes, 'Bridge to Terabithia' is based on a true story. The author, Katherine Paterson, was inspired by the real - life experiences of her son's friend who died suddenly.