There was clearly nothing to do but flop down on the shabby little couch and howl. So Della did it. Which instigates the moral reflection that life is made up of sobs, sniffles, and smiles, with sniffles predominating.
O. Henry evidently wanted it to be understood that Della was not crying because she didn't have enough money to buy her husband a Christmas present; she was crying because this was the last straw. She was crying because of the hard lives she and her husband had to lead on an income of twenty dollars a week, with eight dollars of it going for weekly rent on a shabbily furnished flat. Their plight was symbolized at the moment by the fact that she couldn't even afford to buy Jim a nice present after saving up pennies, nickels and dimes and the whole year. But she was really sobbing for her whole life.
O. Henry often wrote about the hard lives of the lower classes. Some of his best-known stories are tinged with sadness. In "The Furnished Room" a young man commits suicide in the same room where the girl he has been searching for had killed herself in the same way a week before. In "The Last Leaf" the sick girl called Johnsy manages to survive, but the old painter who saved her life dies of pneumonia. "The Cop and the Anthem" is about a man who is trying to get sent to jail so that he won't freeze to death while sleeping on a park bench during a New York winter.
Percy Bysshe Shelley wrote in his "Ode to a Skylark":
We look before and after,
And pine for what is not:
Our sincerest laughter
With some pain is fraught;
Our sweetest songs are those that tell of saddest thought.