In chapter 5, what is significant about Nick's embarrassment during the tea, and the fact that he leaves and walks around the house?

Nick realizes, when he sees the way Jay and Daisy look at one another and when he feels the nervous, awkwardness of the meeting between these two for the first time in almost five years, that his presence is not necessary nor is it wanted.  Nick has good intuition about people because he tends, as he tells the reader in the first chapter, to listen to people, rather than judge them.  Nick is also somewhat stricter morally than either Daisy or Jay (or many of the characters in the story) and it is embarrassing to him because he knows that an affair will take place between the Daisy and Jay.  He tells the reader at the end of the novel that one of the reasons he moved back to the midwest was because he was tired of the immorality of the people in the east.  Also, Nick is too polite and too much the gentleman to eavesdrop or interfere in someone else's business. This scene helps to emphasize Nick's moral character.