Nick attends a small dinner party at Tom and Daisy's house in the first chapter of the novel. What he finds there is unexpected.
Dinner is tense; Jordan reveals that it is Tom's mistress telephoning him, and Daisy appears to know.
In the past, Nick had known Daisy, but now she has changed. At the dinner party the married couple bickers and performs a distasteful drama. Both Tom and Daisy say things suggestive of pomposity, arrogance, and a disdain for culture.
In coming to the east coast, Nick expected something far different from the people there. He expected to have erudite conversations. He expected to encounter mature people and to become mature himself.
When he finds Tom and Daisy involved in a public conflict regarding an affair that Tom is having, Nick is naturally disappointed. The east coast he has found is different and lesser than the one he had hoped for.