How does Martha know that Olive loved her kindness?

She knows about it because Olive's mother shows Martha a page from her late daughter's diary. In it, Olive mentions Martha in a positive way, saying that she's the nicest girl in the whole class. Martha seems to feel that there's a certain connection between them on the basis of their shared love of writing and the ocean. Olive also expresses the fervent hope that one day she and Martha will become friends. However, Martha never used to hang out with Olive when they were at school together. After reading Olive's kind words she deeply regrets that she never got to know her. This mood of disappointment and missed opportunity encourages Martha to write a story about Olive, one that will stand as a fitting memorial to a young life taken much too soon.