Why did Lucy feel comfortable in the faun's cave?

In chapter two of C.S. Lewis' The Lion, the Witch and the Wardrobe, Lucy meets Tumnus, a faun who is in the service of the evil White Witch. In this chapter, Lucy shows her trustingness and naivety by visiting the home of a stranger.
The faun's home is delightful. The text says that "Lucy thought she had never been in a nicer place." It is a little cave with just two rooms: a sitting room and a bedroom. The cave has a carpet on the floor, two little chairs, a table, a dresser, a fireplace with a mantlepiece, and a shelf full of books. This home is cozy and cheery.
Though Tumnus is a spy for the queen, he is a very good and kind fellow. After Lucy becomes queen, she and Tumnus remain friends for the rest of her reign. He serves a simply wonderful tea with an egg, sardines on toast, buttered toast, toast with honey, and a sugar-topped cake. He tells Lucy stories of what life in Narnia is like and plays her music on his flute.
The plot to hand Lucy over to the queen reveals how wrong Lucy was to trust a stranger, but the joy that she enjoys in this cave demonstrates the childlike wonder that Lucy experiences as she explores this new land.