Early in Act I, when Nora and Mrs. Linde are speaking, Nora explains that Dr. Rank "is our greatest friend, and comes in at least once every day." The audience understands from this remark that Dr. Rank knows the Helmers well, and he chooses to see them often. Later in Act I, Dr. Rank arrives, and the audience hears him say to Torvald
No, my dear fellow, I won't disturb you; I would rather go in to your wife for a little while.
The frequency of Dr. Rank's visits, as well as his preference for Nora's company, suggests that he has deep feelings for Nora, but the reason as to why he has them is not clear, until a few moments later, after Krogstad leaves the scene.
After Krogstad leaves, Dr. Rank and Mrs. Linde speak of Mr. Krogstad's altered state, his diseased mind and moral corruption in general. Nora laughs off their serious talk, and according to the stage direction, she even claps her hands:
What do I care about tiresome Society? I am laughing at something quite different, something extremely amusing.
Nora's refusal to engage in the discussion might lead the audience to wonder at her odd timing and her blunt refusal to engage with a solemn conversation, but if Dr. Rank is in love with Nora, her flighty positivity might be what he likes about her. Even though Nora's joy, at this point, is a distraction to conceal her anxiety at Krogstad's visit, Dr. Rank may only see endearing girlish frivolity in Nora, who can, after all, be doll-like in her actions and appearances.