Why does Grandpa believe that Kendra might be the right person to take over the Preserve in the novel, Fablehaven?

When Grandpa gives Kendra the keys, he challenges her to find what they fit, which eventually leads her to the journal that instructs her to drink the milk. The fact that Kendra persists in following the clues makes Grandpa believe she might be the right person to take over the Preserve.