One thing a lot of people seem to struggle with is perspective—and that means other people's.
We see things from our own perspective, the way we allow ourselves to see things, quite readily. But they are quite limited by that.
We're already the hero of our own story, the center of our own universe, but understanding is reinforced by our parents. When we grow up, a lot of us are conditioned by our parents and the other adults who love us to think that we are special and more important than others, because—well, to them, we are.
And there's nothing inherently wrong with this; I'd argue that every child deserves to feel this way: like they're the most important thing in the world. But it brings about a grim realization when you reach adulthood and come to find a world that is utterly indifferent to you. Where your "potential" means much less than what others stand to benefit from you. It's as if the color and saturation are slowly being sucked out of a photograph while you come to grips with a colorless and hard world.
The problem is that some people never reach this realization and go on thinking that they're more important than other people. When they then have kids of their own, they think both that they are the most important and that their children are also the most important. So, the fact that they have children is more important than possibly anything you could ever be, say, or do.
With family members, it's one thing because you'll love your nieces and nephews too, but when strangers or worse, coworkers, do this, it's just plain old selfish and annoying. This coworker is one of those people for insisting that another worker give her his vacation days so that she could have time off with her kids after she "forgot" to ask for the time off when they were on break.
Absolutely, having kids is a lot of work; no one is denying that. But at the end of the day, we all need to learn that we need to plan ahead and take care of ourselves.