She was curious about it and wanted to see what it was like.

Why had Mr.Archibald Craven buried the key?
If he had liked his wife so much why did he hate her garden?
She wondered if she should ever see him, but she knew that if she did she should not like him, and he would not like her, and that she should only stand and stare at him and say nothing, though she should be wanting dreadfully to ask him why he had done such a queer thing.
"People never like me and I never like people," she thought.

"And I never can talk as the Crawford children could.

They were always talking and laughing and making noises." She thought of the robin and of the way he seemed to sing his song at her, and as she remembered the tree-top he perched on she stopped rather suddenly on the path.
"I believe that tree was in the secret garden--I feel sure it was," she said.

"There was a wall round the place and there was no door." She walked back into the first kitchen-garden she had entered and found the old man digging there.