Nick shouts this to Gatsby on the night of his death late in the night.  I believe that Nick said this because he has true respect for him after the accident with Myrtle.  But I also think that he said this because he doesn't appreciate or respect the high-class "friends" of his because they are shallow.  They wallow in sorrow and money and live shallow lives with meaningless material possessions.  Gatsby changed over the story from worrying about money to worrying about people, and that's why Nick respects him.