This article in Newsweek speculates on possible reasons behind scientific studies showing that parents aren't as happy as childless couples. In 2005 Florida State University's Robin Simon, a sociology professor, concluded that having kids doesn't make you happier.
In pre-industrial America, parents certainly loved their children, but their offspring also served a purpose—to work the farm, contribute to the household. Children were a necessity. Today, we have kids more for emotional reasons, but an increasingly complicated work and social environment has made finding satisfaction far more difficult. A key study by University of Wisconsin-Madison's Sara McLanahan and Julia Adams, conducted some 20 years ago, found that parenthood was perceived as significantly more stressful in the 1970s than in the 1950s; the researchers attribute part of that change to major shifts in employment patterns.
The National Marriage Project's 2006 "State of Our Unions" report says that parents have significantly lower marital satisfaction than nonparents because they experienced more single and child-free years than previous generations. Twenty-five years ago, women married around the age of 20, and men at 23. Today both sexes are marrying four to five years later. This means the experience of raising kids is now competing with highs in a parent's past, like career wins ("I got a raise!") or a carefree social life ("God, this is a great martini!"). Shuttling cranky kids to school or dashing to work with spit-up on your favorite sweater doesn't skew as romantic.