The Appeal of the Forbidden Secret:
The Writer's Journey, author Christopher Vogler talks about the Law of the Secret Door (p. 112-113) (also related to Carl Jung's "Forbidden Door"). Many myths include a set-up whereby the heroine is told she must never eat from a certain tree, never open a certain box, or never pass through a certain door, upon pain of death. Of course the myths I’m referring to are Eve in the Garden of Eden, Pandora with her box, and Belle in Beauty and the Beast. We all know what happens, what is sure to happen anytime this sort of situation presents itself in a story. If you have children, you probably have this happen quite regularly in your life as well.
The power of curiosity is universal. In the words of the immortal Dumbledore, "Curiosity is not a sin.... But we should exercise caution with our curiosity... yes, indeed" (p. 598, GoF). Whereas later in the series Harry more strongly develops his own driving need to set the world right by stopping Voldemort, in the first three books, curiosity is one of the prime motivators driving on Harry, Ron, and Hermione.
Threshold Guardians Challenge the Seeker:
At Platform 9 3/4, it is the guard who, with Muggle ignorance of the wizarding world and his belief that Harry is pranking him, keeps Harry from, initially, finding the hidden door. Harry must not give up, he has to listen for clues and approach the odd family that will provide him entry...the Weasleys.