That's why I don't like Rose very much, because of the romance aspect. I can completely understand the companions falling for the Doctor, but he should really know better. But I guess he was feeling very sentimental as Nine when he met Rose - all of a sudden, he was completely alone in the world, and then he bonds with Rose as his companion. So I think it is understandable that he would have felt a very special connection with her.
What I really liked about Donna was that she was completely the opposite of that. She had no romantic interest in the Doctor at all and they were just companions and friends. Completely platonic vibe.

Are you talking about when Donna decided not to go with the Doctor? Because I think that was fairly realistic actually.. what happened to her was really scary and anyone would need time to get over that. You have to be really headstrong to go with the Doctor, especially if you've seen the kind of stuff he gets up to. I know any one of us would not hesitate to go with him and everything, but if that sort of stuff actually happened to you in reality - giant spider monster and her children in the center of the Earth, you would be proper freaked out. Especially if you didn't know the Doctor, like we do. :P