Why is working hard a bad thing? In what world is working hard ever a bad thing?

You're conflating "working hard" with drudgery, or working hard at a job. Praising children for working hard is praising them for their drive. I hope to raise a driven child. Drive is the enabler for acting out your internal desires. I'd congratulate my kid for working hard on solving a puzzle or getting better at playing a game.

I do think that stressing taking care of your mental health is very important. Sometimes downtime is needed.