The movie that made Marilyn Monroe famous was The Seven Year Itch. The idea was that about every seven years most married people begin to itch where they can't scratch. They know something is wrong, but they don't know what, let alone what to do about it. Often they have an affair around that time.
Of all marital problems, that we have confronted the most often in our experience is, "I just don't love him anymore." While there is a vast difference between being "in love" and loving someone, we still crave to be in love.