Fortunately, Jennifer Senior’s new book, “All Joy and No Fun: The Paradox of Modern Parenthood,” is an excellent primer on possible explanations for the great parental pity party. It ranges across the problems with family policy in the United States (basically, we don’t have one), the changing role of children (who went, she notes, “from being our employees to our bosses”), the unsettling of gender roles, the “having it all” stresses inherent in the maternal quest for work-life balance, and the way economic uncertainty and technological change make it hard for parents to figure out what kind of world they’re supposed to be preparing their children for.

But Senior’s most insightful emphasis, I think, is on the gap that’s opened — thanks to our society’s extraordinary wealth and libertarian social ethic — between the lifestyles and choices available to nonparents and the irreducible burdens still involved in raising children.

As she puts it, parenthood is “the last binding obligation in a culture that asks for almost no other permanent commitments at all.” In this sense, it isn’t necessarily that family life has changed that dramatically in the last few generations. Rather, it’s stayed the same in crucial ways — because babies still need what babies need — while outside the domestic sphere there’s been an expansion of opportunities, a proliferation of choices and entertainments and immediately available gratifications, that make child rearing seem much more burdensome by comparison.

This has two consequences for young, reasonably affluent Americans. First, it creates an understandable reluctance to give up the pleasures of extended brunches and long happy hours, late nights and weekend getaways, endless hours playing Grand Theft Auto or binge-watching “New Girl.” Second, it inspires a ferocious shock when a child arrives and that oh-so-modern lifestyle gives way to challenges that seem almost medieval, and duties that seem impossibly absolute. And the longer the arrival is delayed, the greater that shock — because “postponing children,” Senior points out, can make parents “far more aware of the freedoms they’re giving up.”

“Welcome,” a colleague emailed me after our first daughter was born, “to unavoidable reality.” Which is exactly right: In parts of American society, death and children’s diapers are the only unavoidable realities left.