We leave poor Carrie, lonely and sad, in her rocking chair, as the narrator pipes up to address her directly:
Know, then, that for you is neither surfeit nor content. In your rocking-chair, by your window dreaming, shall you long, alone. In your rocking-chair, by your window, shall you dream such happiness as you may never feel. (47.125)
Well that's a pretty grim note to end things on, right?
So, as we've noted in Carrie's character analysis (hop on over to the "Characters" section to read it), Carrie achieves everything she thought she wanted—money, fame—but isn't happy. And at the very end of the novel, she's basically back to where she was at the beginning: longing for that elusive something that will make her happy. Even worse, this line doesn't make us very hopeful that she's going to achieve it any time soon.
It's pretty rare for this to happen to a major character. And it's what makes Carrie a tragic character even though she doesn't die.
And let's not forget that, according to Dreiser, Carrie is our representative "middle-class 19th-century American" so her unhappy ending is probably a bad sign for them as well. In particular, consumer culture at the turn of the century has left Carrie and lots of other folks depressed and brimming with desires that won't ever be quite fulfilled.
Too bad they're all going to have to wait decades for Prozac.