Nick agrees to arrange a meeting between Daisy and Gatsby because he had learned a lot about their past relationship and he wanted to make Gatsby happy.
He was aware of the fact that Gatsby had been loving Daisy for a very long time, and that she was actually the reason why he was living there and why he became rich. He also knew that Daisy still had feelings for Gatsby, and he just wanted them to be together and love each other.