>>537026352
Because Burgerpants doesn't have respect for his job or himself and that annoys Mettaton who has high standards. It's the reverse situation with Tenna where he's annoyed by all his employees who ragebait him all the time, except Mike who is sweet and hard-working.
But you also don't really get to see how Mettaton treats Burgerpants, you mostly just hear what he tells you and he's a very negative person so it might just be his bias.