Why were the mountains dear to him and terrible?

The mountains are dear and terrible to Jody for two reasons. First, he knows little about the mountains surrounding his home. They remain a mystery to him, and since he is not allowed to explore them on his own, the mountains take on an added charm for him.
Second, the mountains are terrible to Jody because they are so imposing. They tower above everything in his surroundings, and all he can see of them are jagged ridges and dangerous cliffs.
To Jody, the mountains are a mystery. His parents know little about them and aren't especially interested in the secrets they hold. When Jody voices his desire to explore the mountains, his father scoffs at the notion. He tells Jody that there is nothing in the mountains worth discovering. Meanwhile, his mother placates him with a reference to a line from a popular children's song: "The bear went over the mountain, To see what he could see."
Jody becomes exasperated and decides to ask Billy, the ranch hand, about the possibility of there being ancient lost cities in the mountains. Billy's answer corresponds with that of Jody's father. These unsatisfying answers further increase Jody's curiosity about the mountains. The mountains are dear and terrible to him at the same time. They represent an unknown but important aspect of his surroundings. Yet, their imposing heights are terrifying to him.