When Grandpa Joe learns that he will be accompanying his grandson Charlie to Willy Wonka's chocolate factory, he is overcome with excitement. He cries "Yippeeeeee!" and "Hallelujah!" and dances merrily around the room.
Grandpa Joe is so excited to go to Willy Wonka's chocolate factory in part because he believes that Willy Wonka is an amazing, fascinating person. When Grandpa Joe tells Charlie about Willy Wonka, at the beginning of the story, he says that Willy Wonka is "a magician with chocolate!" He also exclaims, "Oh, what a man he is, this Mr. Willy Wonka!"
Grandpa Joe also seems to know rather a lot about the story behind Willy Wonka and the chocolate factory. He tells Charlie that there used to be "thousands of people working in Mr. Willy Wonka's factory" and that the factory had to be closed down because it became infiltrated with spies trying to steal Willy Wonka's secrets. The fact that Grandpa Joe knows so much about Willy Wonka and the chocolate factory emphasizes how much he is interested in and an admirer of Willy Wonka.
The other reason that Grandpa Joe wants so much to go to the chocolate factory is simply that he really likes sweets and chocolate. When he talks to Charlie about the chocolate factory, he says that "It makes [his] mouth water just thinking about it." He also becomes very excited when he considers that along with a tour of the chocolate factory each holder of a golden ticket also receives a free lifetime supply of sweets and chocolate. As Grandpa Joe puts it himself, "Just imagine that!"