Why does Nora like to spend time with Dr. Rank?

Dr. Rank is open and honest with Nora and her kindness to him has been interpreted by him as flirting/love. He is dying of a disease, is - in all likelihood - the one who made the loan to Nora, and would help her in any way he could. He is Torvald's best friend, but he will let Nora know that his death is impending when he leaves a card on the table with a black cross drawn on it. He confesses his love to her.