In Chapter 2 of "The Great Gatsby", why does Nick identify with "the casual watcher in the darkening street"?

Nick is our narrator; he is the one who observes what is going on during the summer of the story and reports to us what he saw.  He identifies with the observer in the street because he sees that person as being an objective reporter of the scene. As Nick reports the events of that night in Tom's and Myrtle's secret apartment in the city where they carry out their illicit affair, he is drinking heavily.  Because he was drunk, he tells us earlier in the chapter that everything that happened had a "dim, hazy cast over it".  That is the reason that the narration tends to ramble and jump haphazardly from one observation to the next.  Nick sees the man in the street and imagines that the man sees the row of lighted windows in Tom's and Myrtle's apartment and wonders about what is going on there and in any other lighted rooms. Nick says the man probably is both enchanted and repelled by the variety of life, just as Nick felt that way by the variety of people in the apartment.