It's rude to judge someone else's job.
Paul is at a party where people are introducing themselves when Joe tells everyone he's a janitor.
Paul wants to make a comment about janitors to Joe.
Paul tells Joe that janitors do a hard and thankless job and that he should be proud to be one.
Paul tells Joe that janitors are low class people doing a disgusting and dirty job, and he should be ashamed of his class status.
Joe is happy to hear that Paul thinks so highly of janitors and his self esteem grows.
Joe is insulted that Paul thinks that janitors are below him and angry that Paul thinks janitors should be ashamed of who they are.