Exactly, loving your job doesn't prevent burnout. No matter how much you love it, if you are doing actual work (not some exec shitposting on linkedin all day) then past a certain point your body/mind will just get too tired to function well.

I genuinely love my job. I would do it for free if I could afford to. I sometimes (especially lately) work well over 60 hours a week. But I need to be careful about how much OT I let myself put in because I will burn out. I know that when I push myself too hard I will eventually start fucking up. I will start missing obvious things. I will start making stupid mistakes. With my job I am also far more likely to seriously injure myself when burned out. Allowing myself to become burned out results in worse outcomes for the customers and costs my company more money. Not to mention that if I did injure myself badly enough to be out of work then all those extra hours I put in would be outweighed be the time I miss.

A good manager recognizes that a burned out employee does more harm than good and works to prevent it. A good manager knows that keeping their employees happy, well rested, and fulfilled is in the company's best interest. Sometimes demands pop up that will require a bit of burn out to deal with but the benefit of meeting those demands needs to be weighed against the harm of that burnout. Shitty managers always severely underestimate the harm burnout causes not just to their employees but also to the company.