In Chapter 6, Nick takes on a cautious nature, he becomes Gatsby's confidante, learning more about the man of mystery than he wants to know. He is fearful for Gatsby, knowing how intense the man feels for Daisy. The chapter is narrated with anxiety and worry, there is intensity in Nick's narration in this chapter, the final paragraph conveys Nick's feelings of confusion and fear.
"Through all he said, even through his appalling sentimentality, I was reminded of something, an elusive rhythm, a fragment of lost words, that I had heard somewhere a long time ago. For a moment a phrase tried to take shape in my mouth and my lips parted like a dumb man's as though there was more struggling upon them than a wisp of startled air. But they made no sound and what I had almost remembered was uncommunicable forever." (Fitzgerald)
In Chapter 6, Nick Carraway narrates the truth about Gatsby's background, Fitzgerald gives us this information because Daisy and Gatsby were previously reunited in Nick's home. In this chapter, Gatsby meets Tom Buchanan and both Daisy and Tom attend one of Gatsby's parties.
Nick becomes Gatsby's confidante, he is the only person that Gatsby can talk to about his love for Daisy. And after the party, Gatsby feels that Daisy did not have a good time. For all the mystery about Gatsby, Nick gets to hear the whole truth from the man himself.
I'm not so sure that Nick wanted to be Gatsby's confidante, it made him uncomfortable to be between these two tragic lovers and to have to look at the angry face of Tom Buchanan knowing what he knew.
He tries to be caution Gatsby that as much as he and Daisy felt for each other in the past, that the past is the past.
Nick says: "I wouldn't ask too much of her, You can't repeat the past." (Fitzgerald)
Nick is trying to be the voice of reason in this chapter, he is worried, afraid for Gatsby, he knows that Tom has a really bad temper, and has expressed extreme dislike for Gatsby. Tom does not trust Gatsby, he sees him as a phony, not a real rich person like him, Gatsby is new rich, Tom thinks that Gatsby made his money in bootlegging and has no respect for him.