She had reached the age of seventeen, without having seen one amiable youth who could call forth her sensibility; without having inspired one real passion, and without having excited even any admiration but what was very moderate and very transient. (1.10)
Austen is mocking Gothic literary conventions here, where no self-respecting heroine would exist without some sort of passionate romance. Catherine's relative youth and inexperience are highlighted here as well.
But when a young lady is to be a heroine, the perverseness of forty surrounding families cannot prevent her. Something must and will happen to throw a hero in her way. (1.12)
There's some definite foreshadowing here. While Catherine has been introduced to us as an atypical heroine, the narrative hints here that in this respect she will be like most other literary heroines and will soon meet her 'hero.' Romance is on the horizon.
This sort of mysteriousness, which is always so becoming in a hero, threw a fresh grace in Catherine's imagination around his persona and manners, and increased her anxiety to know more of him. (5.2)
The words used to describe Henry here, "mysteriousness" and "hero," are linked to the emphasis on Catherine's "imagination." These words suggest that Catherine might be viewing Henry as a romanticized, fictional character rather than as a real person.
Every young lady may feel for my heroine in this critical moment, for every young lady has at some time or other known the same agitation. All have been, or at least all have believed themselves to be, in danger from the pursuit of some one whom they wished to avoid; and all have been anxious for the attention of some one whom they wished to please. (10.21)
Catherine finds herself in a love triangle here, a universally understood situation. Given how many ludicrous situations Catherine finds herself in, it is notable that this one is so common. She is pursued by John Thorpe and longs for the attention of Henry.
"You will allow that in both [marriage and dancing], man has the advantage of choice, women only the power of refusal; that in both it is an engagement between man and woman, formed for the advantage of each; and that when once entered into, they belong exclusively to each other till the moment of its dissolution; that it is their duty, each to endeavour to give the other no cause for wishing that he or she had bestowed themselves elsewhere." (10.53)
Henry makes a surprisingly good analogy, or comparison, between marriage and dancing, both of which have certain obligations aside from two people simply loving one another.
"But so it always is with me; the first moment settles every thing. The very first day that Morland came to us last Christmas - the very first moment I beheld him - my heart was irrecoverably gone." (15.12)
Isabella promotes a doctrine of love at first sight here. Given that this is coming from Isabella, these assertions might not be very true. Isabella is greedy, so her idea of love at first sight probably has more to do with money than love.
Once or twice indeed, since James's engagement had taught her what could be done, she had got so far as to indulge in a secret "perhaps" but in general the felicity of being with him for the present bounded her views. (17.1)
Catherine is rather naive about relationships still, and needs James's "example" to show her that marriage might even be possible for herself. It is notable that Catherine focuses on the present though, a trait she carries into other aspects of her life as well.
"Oh! no, not flirts! A woman in love with one man cannot flirt with another."
"It is probable that she will neither love so well, nor flirt so well, as she might do either singly. The gentlemen must each give up a little." (19.15-16)
Catherine's naiveté about relationships shines through here, when she assumes that a woman in love can't flirt with someone else. Given that the woman in question is Isabella, this statement is even funnier. Henry picks up on the irony, of course.
"[…] depend upon it therefore, that real jealousy can never exist between them; depend upon it that no disagreement between them can be of any duration. Their hearts are open to each other, as neither heart can be to you; they know exactly what is required and what can be borne; and you may be certain, that one will never tease the other beyond what is known to be pleasant." (18.28)
Henry outlines an ideal relationship here, where the couple has good communication and a high degree of openness and trust. His final statement about "teasing" one another reveal that Henry's own opinions about relationships, possibly his own with Catherine, are sneaking in here.
his first purpose was to explain himself, and before they reached Mr. Allen's grounds he had done it so well, that Catherine did not think it could ever be repeated too often. She was assured of his affection; and that heart in return was solicited, which, perhaps, they pretty equally knew was already entirely his own. (30.9)
Henry's proposal to Catherine is very sweet and sincere, which may be why we don't get the actual dialogue here. Henry likely reigned in his sense of humor during his proposal. Since this book is a comedy, it may have opted out of printing lines of love and sincerity.