In Act II of Henrik Ibsen's A Doll's House, we find Nora is flirting innocently with Dr. Rank, the family friend, when, suddenly, he expresses her love for her. Although there are not many reasons that Dr. Rank can cite specifically, there is one hint that may lead us to understand his emotions.
RANK:It is just that, that put me on the wrong track. You are a riddle to me. I have often thought that you would almost as soon be in my company as in Helmer's.
From these words, we could argue that Dr. Rank is the type of man that has lived a very conservative life, surrounded by science, and his medical profession, at all times. Notice how, when Dr. Rank explains his illness to Nora, he is bitter about the fact that his disease is common among people who tend to be debauched. Obviously, this is not his case, which is why he is upset that he has to pay for the "sins" of others.
RANK:Oh, it's a mere laughing matter, the whole thing. My poor innocent spine has to suffer for my father's youthful amusements.
Therefore, entering the home of Nora must be a new experience to Dr. Rank. He sees in her everything that he has never seen in a woman, or in anyone, before. Hence, he is able to appreciate, much more than Helmer, the extent to which Nora goes to try and please other people. He obviously cannot understand her personality. Then again, Nora is a riddle to understand, altogether. However, rather than diminish her to the role of a mere entertainer-the way that Helmer has- he actually does see between the lines, and he likes what he sees.