According to Nick what kind of people are Tom and Daisy? Why does he shake hands with Tom at the end of their argument?

From the text:

They were careless people, Tom and Daisy — they smashed up things and creatures and then retreated back into their money or their vast carelessness, or whatever it was that kept them together, and let other people clean up the mess they had made. . . .

I shook hands with him; it seemed silly not to, for I felt suddenly as though I were talking to a child.