It's unpleasant to struggle through something with a team.
You can romanticize the closeness that it will create - the afterglow - but the moment when you test someone's mettle is terrifying.
It's not actually hard on me so much as it's at odds with my self-concept: I want to be demanding but forgiving, supportive, based in trust, respectful of targeted effort.
That said, a sentiment I couldn't help but agree with:
"If I have to choose between a struggling party that's working their a** off or someone slacking, the slacker is easier: just demand they work harder. If they're already killing themselves, you need a more complex prescription..."