The title of A Doll's House refers to the falsity of the Helmers' marriage and home life. Before she leaves, Nora explains to Torvald that she feels she has been living in a make-believe world where he has treated her like a plaything or a child.
Throughout the play, Torvald constantly belittles his wife. He always patronizes her, calling her by diminutive nicknames and doubting her ability to behave sensibly. Torvald is completely, but mistakenly, convinced that he is the only responsible, decisive adult in their home.
In act 3, when Torvald learns of her crime and speaks only of the associated shame of exposure, she realizes that he is not the person she thought he was. When he explicitly states that she has "become both wife and child to him," she announces that she is leaving. She points out to him that they have never before had a serious conversation. Nora underscores the fundamental problem that, rather than actually loving her, he had merely thought that it was "pleasant to be in love with" her.
Nora makes an explicit comparison between herself and a doll concerning the manner she lived with her father. Her father
called me his doll-child, and he played with me just as I used to play with my dolls.
She dismisses her married life as an illusion of happiness in which she “performed tricks” for Torvald. In sum, she states,
our home has been nothing but a playroom. I have been your doll-wife, just as at home I was papa's doll-child; and here the children have been my dolls.