Goodness, what is it with him and attracting all the stubborn, bossy red-heads :|? No wonder he wants to be a ginger; maybe people would listen to him more! But you know, I think he really does like Sophie, despite himself. He likes someone who has a brain but will argue back when something doesn't seem quite right or they don't agree with what he says.

And let's face it. He did sort of develop a soft spot for her while trying to teach her how to dance. I think it was easier to form because he does miss Donna and Sophie's the closest he's going to get to being around someone like her again. (But this time, he isn't bringing her down memory lane or saying Donna this, Donna that like he did with Martha about Rose. He learned his lesson there, albeit after the fact and after he'd disillusioned Martha...)