What do people mean by the "seven year itch"?

Marital restlessness seven years into a marriage became the theme of a 1950s comic movie entitled The Seven Year Itch, starring Tom Ewell and Marilyn Monroe.

There is nothing magical about the seventh year of marriage; people just use the word "seven" because it is a popular symbol in Western culture and mythology (e.g., seven days in a week, seven steps to heaven, seven deadly sins).

There is some truth to the fact that once a marriage has survived past a certain time, it is more likely to continue to survive. In this routine, people who have spent time together, who have accumulated shared memories together, are more likely to prize the continuing tenure of their marriage. Like a job, the more time one has put into the job, the less likely one is to throw it away.

For some couples five years is the "break time"; for others, it might be only two years. And yes, for a few couples, seven years seems to be apropos.