The original question had to be edited. I would suggest that the initial reaction that is had towards the people who attend Gatsby's parties is empathy for Gatsby. It is difficult to not feel bad for someone who ends up doing so much for so many and gets so little in return. Granted that Gatsby has his own motives in trying to win Daisy and the throwing of parties and elaborate display of wealth are ends that he pursues towards that goal. Yet, he is pure of heart in what he is trying to do.
The guests at Gatsby's party cannot be said to feel the same. Conducting themselves “according to the rules of behavior associated with amusement parks,” the guests at the party are there for their own enjoyment with little regard for much else. They drink at Gatsby's expense, use his pool and grounds without much care, and essentially are there to satisfy their own desires. They trash his home, and don't really care about why they are doing what they are doing. The people who attend Gatsby's parties are reflective of the shallow condition of the time period in which people used one another as means to an end as opposed to treating human beings as ends in their own right. They reflect a reality in which people found it acceptable to take advantage of another. It is for this reason that a reaction that is generated from witnessing this is empathy for Gatsby.