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.