Why did Willy Wonka decide to invite five people to his chocolate factory?

Willy Wonka chose to invite five people into his factory because he wants to find someone he could trust to inherit and carry on his legacy. It is ironic that the five people who find golden tickets are all children; because one of the main themes in the story is the bad behavior of children and how it is caused and how it can be prevented. Candy is a sort of metaphor in the story: it represents both a treat and reward, but is also a symbol of greed and gluttony.
Four of the five children come from middle-class or wealthy backgrounds, and are selfish and opportunistic. Charlie's family is very poor and he has grown up learning compassion and generosity. Willy Wonka needed to create situations that would test the honesty and integrity of the children so that he'd know which one of them he could trust with his legacy. As one by one every child failed to pass these "tests" eventually Charlie was the only one who remained. But his success was jeopardized when he and his grandfather broke the rules. Still, he demonstrated honesty and Willy Wonka decided to give him the chocolate factory, which was the fulfillment of Charlie's dearest wish.