I think this paragraph has a lot to do with why Henry Tilney and Catherine Morland work:
Having heard the day before in Milsom Street that their elder brother, Captain Tilney, was expected almost every hour, she was at no loss for the name of a very fashionable-looking, handsome young man, whom she had never seen before, and who now evidently belonged to their party. She looked at him with great admiration, and even supposed it possible that some people might think him handsomer than his brother, though, in her eyes, his air was more assuming, and his countenance less prepossessing. His taste and manners were beyond a doubt decidedly inferior; for, within her hearing, he not only protested against every thought of dancing himself, but even laughed openly at Henry for finding it possible… Catherine, meanwhile, undisturbed by presentiments of such an evil, or of any evil at all, except that of having but a short set to dance down, enjoyed her usual happiness with Henry Tilney, listening with sparkling eyes to everything he said; and, in finding him irresistible, becoming so herself.
Henry is a second son, and not only that, his brother is HOT. Smoking hot, and from what we hear of him later, pretty charming, though not funny charming. Catherine takes one look at him, is like, “Well, other people might think he’s handsome,” and then focuses on Henry.
How often has this happened in Henry’s life? Is the usual way of things that a girl dances with him, finds out who he is and tries to trade up for his brother, Isabella-esq? I can see that happening to a second son, a mere clergyman A LOT. But the thought doesn’t even enter Catherine’s head, she’s all about Henry. She’s loyal, she’s disinterested, she’s not a mercenary. How gratifying it must be to be her first and only choice!
Is it any wonder that he falls in love with her? I say no.