Nick has begun to realize how phony it all is. He understands that most of the people in this social class are just going through the motions. There is nothing real about them.
I think alot also has to do with the fact that Gatsby, unlike Tom and Daisy, knew what he wanted, was a driven man, and was willing to go to great lengths to be happy and to make Daisy happy. He did not want to continue this charade any longer and wanted Daisy to tell Tom the truth. However, Daisy does not know the truth and, though she claims that she truly does love Tom, there really is no way to know if that is the only reason she will stay with him. She has stayed with him through all of his cheating and deceit, so perhaps she does love him or perhaps she is simply much too fearful of being without anyone. It seems she just wanted to stay married to Tom and keep Gatsby on the side, much like Tom wanted to stay married to Daisy and keep Myrtle on the side. Neither one of them wanted to be honest with the person they supposedly cared about. Gatsby, however, wanted a true relationship with Daisy and, perhaps because of his humble beginnings, was well aware of the sacrifices loving someong at times entailed; he was willing to do that. Therefore, Nick's statement shows that he appreciates Gatsby's openness and love for Daisy.
In the beginning of the novel, Nick expresses disdain for the kind of people he sees Daisy and Tom as: the idle rich, playing with their lives and other people's lives because they're bored. He originally put Gatsby in this group of the idle rich. By the end of the novel, Nick has learned that all Gatsby acquired and did was to impress Daisy, and that beneath that rich, mysterious exterior was a guy who truly cared about Daisy.