Once it was simple. You got married, had kids, worked the land, and stayed married whether you could stand each other or not. The concept of "a happy marriage" was no more relevant than the idea of "a pretty tractor."

"That has changed over time as marriage has become more independent," says Steven Nock, a professor of sociology who studies marriage at the University of Virginia and author of Marriage in Men's Lives. "Couples don't need each other for quite as many things as they once did. If you're running a farm with someone, it doesn't matter if you're pissed at her or not. You need her labor as much as she needs yours. The couple is more or less equally dependent on each other."

Chances are, though, if you are reading this, you are not running a farm with your mate. And if you are, you are probably doing it out of choice, not necessity. As a recent Washington Post story pointed out, "As marriage with children becomes the exception rather than the norm, social scientists say it is also becoming the self-selected province of the college-educated and the affluent." Marriage in America is becoming more like a luxury car ―― in other words a BMW, not a Harvester.

This doesn't necessarily mean modern marriages are happy marriages. According to the latest U.S. Census data, the overall divorce rate has declined as couples get married later in life, often after living together. But the divorce rate for first marriages is still about 47%.

Having a happy marriage today means thinking of reasons to be together

"From my perspective, the hardest thing is issues of commitment and trust," says Nock, who has followed couples over time and conducted interviews with 6,000 married men since 1979. What does commitment mean to the modern husband? "I'm going to behave myself because I'm committed to this relationship," is how Nock describes it. Because people have left the farm, and because women have achieved financial parity, married people need new reasons to stay together.