I'd like to add another less admirable motive to Helen
Batterby's analysis of why
Henry Crawford genuinely seeks Fanny's hand in
marriage. I agree we are to take the strongest motive
to be Fanny's withholding of herself from him. The
scene at the Parsonage at dinner (which Mrs Norris tries
so hard to poison for Fanny) is shaped in such as way
as to call attention to one small moment in it, the
moment when Fanny contradicts Henry and says how
relieved she was when the play was over; Austen
then follows this up by the dialogue between Mary and
Henry wherein Mary agrees to stand by while Henry
works at making Fanny trail after him publically,
saving seats for him in public and generally eagerly
jumping to his call: what he says there is what galls
him is she has through her manners told him she
does not, will not, and shall not like him, so there.
There are some admirable understanding motives which Henry brings forward. He shows himself to have become intensely aware of her intelligence and kindness of heart, and, interestingly, her passionate love for her brother. It would be something to have such a love directed towards himself, and he values her. He could look forward to happiness in marriage with such a woman because once in love she would always yield, and yield to him an intelligent perceptive and loving heart and mind. Thus he expatiates on her good temper, her forbearance, her understanding, her elegance of mind (as opposed to manners), his integrity, and his understanding that he would trust her. She would not lie (II:12, 294). In a way this portrait harkens right back to Anhalt's portrait of what makes a happy ideal marriage in Lover's Vows.
All this Henry consciously presents to Mary in his attempt to tell her that now his "project upon her [Fanny's] peace" (Chapman II:12, 295) has become a project to give her greater peace than she can ever know as someone (in his words) "dependent, helpless, friendless, neglected, forgotten." Henry has a heart capable of pity too. Here Henry reminds me of ASByatt's Euan McIntyre, a suave well dressed witty and rich upper class banker who falls in love with Val, working class in origins and now despite a Ph.D., a lowly gauche secretary working for a Temporary Agency, and why, because while she was at first to him a sort of kitten who had no claws so he thought he'd amuse himself with, she became instead a creature whom he wanted to protect, support, give a rightful place to because she touched his heart.
Mary does not quite get all this because she hasn't the reflective powers to appreciate them. She is mostly amused, and we are told "not displeased wioth her brother's marrying a little beneath himself" (II;12, 292). She takes the world's view; she is not seeing or feeling in the deeper way Henry is in this chapter; to her-as it would be to the Mrs Bennets of the world--Fanny is no trophy because she has no great dowry & no connections beyond her uncle. But as Henry has now married down, maybe this will excuse _her marrying down (how she views marrying a second son who is to be a clergyman). But she is also generous. Let us imagine Miss Bingley's response, or in some moods Emma's. She is glad for Fanny; no-one deserves worldly elevation more. There is a kind of irony here when she says no-one deserves Henry more; as Henry cannot see the Admiral and Fanny would never get along so Mary cannot see Henry's philandering idle selfish ways up until now are precisely what Fanny does not deserve to have to endure--or indeed know how to handle.
But Mary, as a woman not blinded by self-love (as Henry in part is), also has a sense that Fanny might not leap at the offer: She has a feel for Fanny's personality as such; Henry says he knows she is not like Maria or Julia, but he doesn't fathom what lies behind Fanny's withholding in the intuitive way Mary does. Mary says warningly: Fanny would not marry "you without love; that is, if there is a girl in the world capable of being uninfluenced by ambition, I can suppose it her;" still "ask her to love you, and she will never have the heart to refuse" (II:12, 293). Mary is oddly sentimental here, and she speaks out of her own love and loyalty to her brother (one of her best traits in the book).
But there is a less sweet and not really admirable motive which neither Henry nor his sister are quite conscious of, and this like the one Helen mentioned has to do with triumphing and conquest. Henry wants to triumph and conquer Fanny not only because she has refused to yield easily but because in doing this he buys a sign of his inward worth. Fanny is in fact wanted as a trophy wife; by having Fanny at the center of his life he shows to the world what a valuable man he is; he proves to everyone "what sort of women it is that can attach me, that can attach a man of sense" (II:12, 297). It's true the idiots of the world will not admire his choice, but the finer spirits whom Henry can appreciate will understand that he Henry has things in him others have not been aware of. The fact that Fanny is "not like" her cousins, and his understanding of all her inner qualities is his way of signalling to those in the world who can understand that he has them too. Most conventional people think of trophies as that which compels the admiration of the average person; Henry is above this.
Curiously he brings up the Admiral in this connection to. He will prove to the Admiral there is such a woman. This is interesting because it looks forward to what Henry might have become had he married Fanny and his love not turned to boredom or his personality begun to alienate her. Since marriage was "forever" in this period years later (the ten Mary mentions) might indeed have seen Henry less restless and happy with this trophy wife he can depend upon and who will be a helpmeet to him.
But it is the triumphing over the world, proving himself to it itself that Austen wants us to see. Henry is not going to go to sea and win respect that way. There is no job out there for him, and, as we shall see, unlike Knightley, it is not enough for him to be a good landlord for its own sake. He wants acknowledgement. And he will win this and the respect of those he is subtle enough to understand have a respect worth having by showing everyone what good taste and perception and appreciation of a finer person he has and how far his money can go:
"What can Sir Thomas and Edmund together do, what do they do for her happiness, comfort, honour, and dignity in the world, to what I shall do?" (11:12, 297).
To save a tenant from a rapacious steward (as he plans to do if someone will only cheer him on and say how wonderful) is still not much and maybe even nothing; to save a heroine, well.
One could say that MP is a revision of Lover's Vows seen in a deeper way than most writers are still not capable of.