John Kennedy Jr. and Carolyn Bessette arrive at the New York City Breast Cancer Foundation Luncheon, New York City, Oct. 20, 1998. (Patrick Mcmullan/Getty Images)

What was the biggest source of marital strain for John F. Kennedy Jr. and wife Carolyn Bessette Kennedy?

One word: Paparazzi.

In her new book, "Fairy Tale Interrupted," JFK Jr.'s personal assistant and confidante RoseMarie Terenzio reveals that the simmering tension between the Kennedy scion and his glamorous wife was the product of those never-ceasing flashbulbs.

RoseMarie Terenzio and John F. Kennedy Jr. (Courtesy RoseMarie Terenzio)

Long accustomed to the media following his every move, the son  of President John F. Kennedy  wasn't fazed by negative coverage. But his wife wasn't nearly as thick-skinned, and when Bessette Kennedy complained, she faced blowback from her husband.

"It made him angry. He would say to her, 'Oh, big deal, so what? Get on with it," Terenzio told "20/20's" Chris Cuomo.  "And sometimes that was hurtful."

In her book, Terenzio wrote that Kennedy's insensitivity was "the biggest catalyst for their arguments."

RoseMarie Terenzio, left, eventually became a confidante of Carolyn Bessette Kennedy too. (Courtesy RoseMarie Terenzio)

The nastier the press coverage got, she wrote, "the more Carolyn retreated into herself. The process was heartbreaking to witness."

Watch the full story tonight on "20/20? at 10 p.m. ET.