In The Great Gatsby, why does Nick become offended and refuse Gatsby's business offer?

At the beginning of Chapter 5 in The Great Gatsby, Nick becomes offended and refuses Gatsby's offer to share in his "little business" because "the offer was obviously and tactlessly for a service to be rendered" (83-84).  In other words, Nick is perturbed that Gatsby thinks his bootlegging job offer to be a way to repay Nick for allowing the first meeting between Daisy and Gatsby (after many years) to be at Nick's little home.  In addition, it probably didn't help things much when Gatsby (again quite tactlessly) keeps saying things like, "Old sport, you don't make much money, do you?" (83).  I wouldn't go so far as to say Nick was "offended," however.  Heck, in the very next paragraph, Nick says, "The evening had made me light-headed and happy" (84).  Nick is happy (and a bit excited) to help Gatsby, for Nick is discovering Gatsby to truly be the great Gatsby that the book title speaks of (his bootlegging side-job aside).