The most evident and consistent trait that can be drawn from the character of Nora is her insistence in trying to draw from those around her either a maternal or a paternal treatment. She enables this behavior by acting like a child. Yet, from the manner in which she obviously pushes down her reality (this, being the bad dealing that she did with Krogstad), and the potential ruin that she could bring to the good name of her family, we can attest to the fact that Nora's life has been one where denial takes the place of reality.
It seems that, from a very young age, Nora has been trained to act in a way that would please everyone around her. She, herself, admits to having been treated like a doll by her own father. When people are conditioned to behave in a way that does not become them, or should not become them, they are prone to continue doing things against their will automatically. This means that Nora, automatically, pushes down reality and acts, plays, sings, dances, and entertains instead of doing what she is supposed to do. Far from being silly, she is simply doing what she has always known how to do.
I have been your doll-wife, just as at home I was papa's doll-child; and here the children have been my dolls. I thought it great fun when you played with me, just as they thought it great fun when I played with them. That is what our marriage has been, Torvald.
The Christmas tree scene is interesting because, as a woman- child, Nora is enthralled by the magic of Christmas while also living in despair and in fear for what could happen to her if Torvald finds out about the Krogstad deal. Yet, just like a child, she prefers to brush away the immediate concern and gives more weight to the immediate pleasure of decorating and having fun. This seems to be the sweet and sour nature of Nora's life: an adult with real problems consistently acting with the naivete and joy of a kid. In Nora's case, this leads nowhere.
In the end, the sour will prevail. Even after she has confronted the reality of Krogstad now she is about to face the one truth that forever remained buried: that her strength of her marriage is fake.