I was going to say the same thing - there are two or three different times he's called "a freak" and he defiantly, but quietly says "No, I'm not. I'm not..." If he were a true sociopath and misanthrope, he'd have laughed at that or not even batted an eyelash. But there is definitely an element of wanting to be appreciated for exactly what he is. And he's not. And that is the loneliest thing there is.
I'm not sure that would be the motivation. IMO the Joker's denial of being a freak would be fairly typical of a sociopath who's not in the mood to hide it.
At one point, the Joker remarks that he prefers to kill with a knife rather than a gun because guns can kill quickly and without personal contact -- and because, he says, at the moment of death 'people act the way they really are.' (Sorry, this is a paraphrase -- have only seen the movie once so far.) In some ways, that summarizes his relationship with the rest of the world.
Enough pain and stress and terror can reduce anyone to acting on their basest survival instincts. That's not
all we are, but to a sociopath, those lowest common denominators are what define a human being. And a violent sociopath like the Joker could be convinced that he's the only one around who's superior enough to come to terms with that and even to celebrate it via spreading chaos. In other words, the Joker is convinced that he's not a freak because he's convinced that everyone is just like him -- only less smart and more in need of illusions, which essentially makes them (in his mind) inferior beings to be disposed of in whatever way he finds entertaining.
That's an thumbnail of my reaction to this character -- I saw TDK for the first time yesterday and am still thinking over what I want to post about it.