This. There have been a number of articles written about why the mantra to "do what you love" can be a bad one, and this is one of the reasons. If you're passionate about what you do, many employers will exploit this fact. You'll end up being one of these chumps who works 80+ hours a week, sleeps on the couch in the office, and subsists on leftover Chinese takeout reheated in the office microwave, cold pizza, and Mountain Dew.
Once people get older, they also develop other priorities: a spouse, kids, aging parents, health problems of their own caused by a couple of decades of lack of sleep and eating crap food and not exercising. They realize that no one ever dies wishing they'd spent more time in the office. They start to establish boundaries around their working life so that they can engage in better self-care, and having meaningful relationships with the other people in their lives. It doesn't mean that they're not passionate about what they do any more; it's a sign that they're no longer willing to allow someone else to exploit that passion to another person's profit rather than their own.