quizcustodet read Sense and Sensibility recently and talking about it with him revived all my extreme Colonel Brandon hate. So I'm going to rant about him for a while under a cut, just in case anyone hasn't read it and intends to.
What it boils down to for me is that he constantly infantalises women and I think Marianne deserves better. As far as he's concerned, nothing is ever the responsibility of a woman. She is always a victim of fate.
His first love didn't have enough backbone to refuse to marry his brother but, rather than be annoyed with her about this, he sees her subsequently running off with some random bloke to be something that somehow just happened to her and that she had nothing to do with. I'm sure that being in an unhappy marriage sucks but that doesn't take away the fact that she chose to deal with that by running away from that. If he were simply annoyed that her husband could do what he pleased but what she did ruined her, I would have quite a lot of sympathy with him but he doesn't seem to think that she did anything actively at all, she was just passively ran off with. Hmmm...
The same thing happens to his ward and, again, that's something he sees as something that passively happened to her, not something that she chose to do. There's even less justification for her than for her mother - she's well provided for by Brandon, he's not going to force her into an unhappy marriage, I see no reason to suppose that she didn't choose to run off with Willoughby. If he promised her marriage or some such to seduce her, that would be one thing (although it would still be deception and not force) but there's no evidence to suggest that that was the case. Willoughby seems to be rather careful who he promises marriage to.
The main reason why he isn't interested in Elinor seems to be that she's a rational adult and he couldn't realistically see her as being passive. But he can see Marianne as a(nother) victim of Willoughby that he can rescue. Despite the fact that she was never really a victim there. As Elinor constantly points out to her, she is rash in the way that she lets everyone see her feelings for Willoughby. She had the choice to be more circumspect, that choice was constantly put in front of her by Elinor, but she made the risky choice to put her heart on her sleeve. She sees that and acknowledges that she would have been a better person if she had followed Elinor's example but Brandon doesn't. He doesn't really see anything wrong in anything she's done, largely because he doesn't really think her capable of choices and actions. Marianne is in a silly teenage phase but she deserves better than someone who thinks she's a child. Since she is highly likely to grow up, Brandon will probably get bored of her and develop a crush on someone young enough to be his grandchild next anyway..