When I say I'm dissatisfied with him, it's not because I dislike his personality or anything like that. I can't help but think that he doesn't deserve Fanny because of the way that he treats her. Even though he cares about Fanny & has been the only one at Mansfield to be really kind to her, it only takes a pretty face the shallow but witty chatter of Mary Crawford for him decide that suddenly, Fanny's wants and needs must be less important.

I think it actually makes it worse that he was the only one before who was kind to her, because it means that she's so unused to being treated the way she deserves that he gets away with it. She feels gratitude just for being remembered at all, even when she's feeling pain for having been forgotten before.
Also, I don't like his way of talking to here, even before Mary Crawford. Yes I know he's kind to her, yes I know he encourages her, yes I know he helps draw her out sometime when she needs it--but he also talks down to her. He doesn't really listen, except insofar as he wants to see if she agrees with him, and he dismisses her objections to the Crawfords, assuming that since he doesn't see it she must be imagining things.
So altogether it's not his personality that bothers me--his personality is fine, and I can completely understand why, based on that (and on their history) Fanny is in love with him. But I wish that somehow she stopped loving him so much above everything else when he starts treating badly because of Mary Crawford-- or I wish at least that she found someone better, someone who really loved and cherished her like she deserves, & so deserves her love back.