Why do you think Della flops down on the couch and howls?

This is a graphic way of showing how Della is feeling. She is painfully distressed because she can't afford to buy her husband Jim a nice present for Christmas. As O. Henry reiterates, she has only managed to save $1.87 all year, and tomorrow will be Christmas. 

There was clearly nothing to do but flop down on the shabby little couch and howl. 

Even in those early times around the turn of the twentieth century, there wasn't...

This is a graphic way of showing how Della is feeling. She is painfully distressed because she can't afford to buy her husband Jim a nice present for Christmas. As O. Henry reiterates, she has only managed to save $1.87 all year, and tomorrow will be Christmas. 

There was clearly nothing to do but flop down on the shabby little couch and howl. 

Even in those early times around the turn of the twentieth century, there wasn't much you could buy in the way of a present for such a small sum of money. She doesn't know that her husband is having the same problem. He would like to buy her something especially nice for Christmas, but he can't afford it. He only earns $20 a week, and eight of it goes for rent on their flat.

Flopping down on the couch and howling shows Della's desperation. Her desperation makes her come up with a radical idea. She will sell her hair! Jim comes up with a comparably radical idea. He will sell the gold watch which has been in his family for three generations. We don't really know what is going on in Jim's mind because we are confined to Della's point of view, but we can imagine what he was thinking and feeling during his long day at the office.

These two impoverished people need something to cheer each other up at this time of year—and something to cheer themselves up. Something special. Something that suggests future prosperity. Something to make other people think they are doing better than they really are. Jim will have a platinum fob for his gold watch, and Della will have a set of high-quality combs to adorn her beautiful hair.

The story is full of sadness, and the ending only seems to make matters worse. Della's hair and Jim's watch are both gone. Yet O. Henry manages to end on a bright Christmas note. What is really important is that they still have each other and are both still in love.