Why was Steerforth sad and angry with himself in David Copperfield?

When David begins school at Salem House, James Steerforth takes him under his wing (Chapter 7). David is enthralled by Steerforth’s winning ways and cannot see how selfish and manipulative the other boy is. Steerforth, whose family is well-to-do, has a superior attitude that extends beyond the other boys and to the teachers.
He learns that Mr. Mell is not only poor himself, but has a destitute mother who lives in an alms house. One Saturday in class, Mr. Mell loses his temper when the boys are goofing off. Steerforth, who is whistling, talks back to Mell when the teacher tells him to be quiet. Mell also calls him out for exercising too much influence over David. Steerforth gets embarrassed and then angry, and in response to Mell's further criticism, calls the teacher "an impudent beggar."
https://etc.usf.edu/lit2go/166/david-copperfield/2941/chapter-7-my-first-half-at-salem-house/