This passage is Torvald's answer to Nora in her apparent worry over a revenge attempt from Krogstad -a reasonable assumption considering that her father went through the same situation with that result. Torvald gives very little credence to Krogstad because of his opinion of him, belittling him and reassuring that he was a big enough man to take anything Krogstad threw at him. He also views her worry as a proof of love for him, implying he still needs some to believe that she does. Any emotion that was in her worry was out of self-preservation rather than love. In reality, it wasn't for his sake she was worried, but her own, as Krogstad told her before that any attack he would make would be against her and her forgery. Torvald had just sent a notice to Krogstad informing him of his firing, ending any chance she had at convincing Torvald not to fire him. This also closes most of the solutions she had on fixing her problem, making her more desperate and hysterical. Later in the story -once all pretenses are gone and her secret is revealed to him- she becomes very calm, as she no longer has to work around him and can get straight to the heart of the matter rather than finding a roundabout way of asking him or needing a way to side-step him. Anything that's on her mind she can say freely, and does so, revealing her true intentions this entire time.