It never really fully explained in the book. You have to pay close attention to details and little things that nick did and said about wealthy people in the book and if you have watched the movie it gives you a better understanding. Nick doesn't agree with how most wealthy people act. If you can't act right and be wealthy then he disapproves that's just his opinion about it. Hope that helped!
Nick disapproves of Gatsby's wealth and status because he views it as morally corrupting, leading to isolation and superficiality. He is critical of the extravagant lifestyle of the wealthy and reflects on how it diminishes genuine human connection. Additionally, he sees Gatsby's life as a tragic twist on the American Dream, one that does not lead to true fulfillment.
;