When scientists studied the brain chemistry of the newly love struck they found that certain chemicals are elevated when love is new. Researchers at the University of Pavia, for instance, found that levels of nerve growth factor (NGF)—a protein that maintains the health of neurons—were higher in people who had reported just falling in love when compared to single people or those in long-term relationships. After about a year, though, the subjects' NGF levels fell back to a normal level.

But after the first years of wedded bliss, some discontentment seems to follow. A poll of 5,000 married couples found that men and women begin to take their marriage for granted after two and a half years. Half the couples surveyed for the 2008 study reported that they felt undervalued at the 2.5 year mark. The majority of the men said they stopped picking up after themselves, while the women were no longer making an effort to look nice for their spouse. A 2011 survey of married couples found that irritation peaks at the 3-year mark. More than two-thirds of all of those surveyed said that little quirks, which were seemingly harmless and often endearing during the first flushes of love, became major annoyances at 36 months.