The Seven Year Itch - inclination to become unfaithful after seven years - is the title of the film from the 1950's starring Marilyn Monroe.
In the UK the average length of marriages ending in divorce is actually over eleven years ( in 2007 there were 270,000 marriages and 144,000 divorces ).
It can't be explained on a cellular level. It's true many cells die + are replaced by stem cells which contain all the genetic information needed to produce identical tissues. With skin and gut, this is rapid, other tissues slower, and some cells ( in brain and heart ) can't be replaced. There are controls and balances, including the action of hormones like insulin, thyroid and steroid hormones. Germ cells ( sperm and eggs ) allow genetic information to be modified and passed on to the next generation.
Errors in cell division may result in disease like cancer. But we don't become "totally different person(s) ". There may be other reasons for not sustaining love and interest in each other beyond a decade or two. In the past people married younger, had children, and stayed together until " death us do part ". Now with increased life expectancy people live several decades after marriage, children have left home, emotional and economic benefits may decline, and new opportunities exist ( like internet dating ). This is not to condone what is happening, just to explain the facts.
The original " seven year itch " was a phrase used in the USA from the 19th century to describe a skin condition which LASTED seven years - probably scabies, which can now be effectively treated
