Northanger Abbey has a remarkably lighthearted and affectionate mood. The narrator often pokes fun at many of the characters, and the story never becomes serious or didactic—rather, it remains in the realm of humor and fondness. The novel also encourages readers to understand and sympathize with the little joys and dramas of each character.
For example, the reader develops a fondness for Catherine despite her many foibles. In Volume 2, Chapter 7, she shows an endearing capacity for wonder:
Catherine had seen nothing to compare with it; and her feelings of delight were so strong, that without waiting for any better authority, she boldly burst forth in wonder and praise. The General listened with assenting gratitude; and it seemed as if his own estimation of Northanger had waited unfixed till that hour.
Of course, the General does not really care about Catherine's assessment. But he does respond with disproportionate joy to any praise of his estate; so, he only esteems her opinion because it aligns with his own. Readers also get a better sense of Catherine's endless wonderment in this passage, as she "boldly burst[s] forth in wonder and praise." Granted, some of her sources of amazement might be somewhat underwhelming to people who have a bit more life experience than her, but this doesn't take away from the fact that she appreciates the beauty of Northanger Abbey in a charming and innocent way. This contributes to the novel's lighthearted, affectionate mood.