Why do you think Lupin reprimanding Harry for using expelliarmus on Stan Shunpike stikes such a nerve for Harry and results in him getting rather defiant?

I think part of it is that he respects Lupin and doesn't want to be criticised by him. Part of him feels he was right to use it but another part feels stupid and he says Lupin was also reminding him when Zacharias Smith was so dismissive of him using that spell