Why is Mr Knightley getting so very worked up about Frank Churchill's failure to appear in Highbury?

Does he in fact disapprove very strongly of selfish young men who fail to do their duty?

Does he get drawn into the argument because of exasperation at Emma's opposition? It seems to me that Emma is sparring/disputing with him for the sheer pleasure of the argument, rather than because she has particularly strong feelings about the subject.