After talking with many married men and women, we came up with a few theories to explain this. One is that women are still doing more than their fair share of domestic chores, even if they're also earning a substantial portion of the family income. As one mom, Sandy, said, "I come home from work, I'm tired, and it's still up to me to get dinner ready, oversee homework, and get the kitchen cleaned up. It's really hard to paint a smile on my face and lovingly say, 'Hi honey! How was your day?' when I'm in the midst of it all."
Another reason why men are happier than women is that women tend to have higher expectations for their happiness, and are therefore more likely to fail to reach those standards. (How insidious is that—being even more miserable because you've failed to reach your happiness goals?) Women are also more prone to link their happiness to things outside of themselves. I'll be happy if my husband comes home and thanks me for making dinner. I'll be happy if my kids behave themselves well. I'll be happy if I can watch an entire episode of Grey's Anatomy without having to get up. Then, if those things don't happen, we're not happy, when it might have been possible to be happy if we hadn't pinned our joy to external things.