Kino has a traditional provider mentality, in that, he sees others as a block to his success and riches and therefore treats them with suspicion. Because of his way of seeing things, he is competitive. He finds a rare pearl and immediately comes to the conclusion that everyone who knows about it will want to steal it or trick him into selling it at a low price. Throughout the story his obsession with the pearl distracts him from the illness of his infant son. In the end, he kills his own son because of the pearl. It's not really because of the pearl, itself, but what it represents--what people want to take away from him. The moral of the story is that which is truly our own--the child--cannot be taken away.
Juana has a more primitive/spiritual mentality. She is aware of how negative thoughts and actions can lead to a negative outcome. She sees the problem of the pearl as a lesson about human greed and spiritual truth. For example, even at the beginning after the doctor leaves when Kino wants to hide the Pearl again she thinks it is a harbinger of something bad to come. Nearer to the end of the story she takes off with the pearl and tries to throw it in the sea. At this point she really believes that the presence of the pearl is what is causing her baby to be sick.