The answer to that is not spelled out in the story, but her despair at the end of the story indicates that she has accidentally learned just how empty life is. To be specific, when she gets drunk, she doesn't seem to care who she dances with. That means that she's not with her boyfriend out of love, but out of…habit. Default. What's more, this man's life seems to be pathetic and empty, but she slides right in, just like she belongs there. Very sad.
I think it's like what the garage sale represents; both the couple and the man are both unwanted, lost, useless and lifeless. Adding on to what gbeatty said, it really does give us a sense of emptiness in both characters, and how commonplace their life is.
It doesn't matter if you're young, or if you're old, bitterness is universal, and the young couple blends right with the old, pathetic "middle-aged man."
I think the ending is the most, I don't know, in-your-face part of the whole story. It's just, you get such a pleasant feel from the man, and you really can see the warmth and the semi-intimate atmosphere of it all, yet they just treat it like a weird story to see.