The New York Times reported on research by Kelly Musick and Larry Bumpass regarding marriage and happiness. Maybe not surprising to anyone who's been married, they found a "honeymoon" of maybe 3 years followed by a downward dip in cross sectional data.
What is interesting is that over time, couples who stay together may get better at moderating each others' emotions. Robert Levenson at UC Berkeley pursues research in this area. Key issues are who displays these kinds of balancing characteristics and why.