In Act 3 of A Doll's House Christine Linde and Nils Krogstad reunite after years of separation. Many years before, they were in love and even planning to marry. However, Kristine had a lot of responsibilities to fulfill at the time. A marriage to Nils would have been cumbersome since he had no money to ease Kristine's situation
You must not forget that I had a helpless mother and two little brothers. We couldn't wait for you, Nils; your prospects seemed hopeless then.
As a result Christine marries another man for convenience, and not love. She never achieves happiness. Now a middle-aged widow whose children are gone from home, Christine refuses to let go of what could be her last chance to feel happy, useful, and renewed again. She wants to be a mother to Krogstad's children. She also wants to regain a sense of life. She really has nothing to lose anyway; she is alone in the world and whatever she would risk would be very little. Like she says, she, too, is a shipwreck just as Krogstad. It is best to get together and give each other a sense of purpose.
Two on the same piece of wreckage would stand a better chance than each on their own.
In Krogstad, Kristine has found her one chance for happiness. We know that they had a previous relationship and that Kristine married someone else. Kristine did not marry for money; she married someone who could help her take care of her family. The marriage was more for financial reasons than anything else. When Kristine is reunited with Krogstad, she remembers the man he was, and he longs to be that man again. Krogstad and Kristine both find a chance at happiness with each other. This happiness is in direct contrast with the misery and lies that characterize Nora and Torvald’s marriage. If you think about it, what does Kristine really have to risk? Perhaps her job at the bank or her friendship with Nora? Neither of those are even on par with the happiness that she might find with Krogstad.