There are two reasons why Della was upset at the beginning of the short story. First, she was upset, because she was poor. She saved as best as she could, but she did not have enough money. Here is what the text says. In fact, it is the opening words of the short story.
One dollar and eighty-seven cents. That was all. And sixty cents of it was in pennies.
Second, it was Christmas. This is the more important point. Della wanted to get her husband, Jim, a present to express her love. So, she pondered what she could do. In the end, she chopped off her hair, her most prized possession, and she sold it to gain money for a present. The important point to underline here is that Della wanted money not for selfish reasons; it was just the opposite. She did this to express her love in a selfless way.
At the end of the story, her sacrifice and her husband's sacrifice bring joy to each other. So, she may have started sad, but in the end it was transformed into happiness.
Here is the moral of the story:
The magi, as you know, were wise men--wonderfully wise men--who brought gifts to the Babe in the manger. They invented the art of giving Christmas presents. Being wise, their gifts were no doubt wise ones, possibly bearing the privilege of exchange in case of duplication. And here I have lamely related to you the uneventful chronicle of two foolish children in a flat who most unwisely sacrificed for each other the greatest treasures of their house. But in a last word to the wise of these days let it be said that of all who give gifts these two were the wisest. O all who give and receive gifts, such as they are wisest. Everywhere they are wisest. They are the magi.