Brad and Angie’s split had been ‘brewing for a long time’

Los Angeles - Brad Pitt and Angelina Jolie's divorce had been brewing for a long time.

The By The Sea actress filed for divorce from the 52-year-old actor on Monday citing "irreconcilable differences" but it has been claimed that the pair were having difficulties for a while.

A friend of the pair said: "I could say the divorce announcement is a shock, but it’s not. This divorce deal has been brewing for a long time.

"When Angelina underwent her mastectomy in 2013, her health and life in general became a priority for her. Her views on life changed. Suddenly she saw herself walking in her late mother’s shoes - and she was no longer that wild, fun loving woman she used to be, she was taking things a lot more seriously.

"Brad was supportive, and wanted to be there for her, so they decided to marry - more for the sake of their six children than themselves. They wanted the kids to have the security of knowing their parents were committed because they had been very affected by their mom’s ill health."

WATCH: Brad and Angelina Jolie's last interview together

The couple - who had been together for 10 years before they decided to tie the knot in 2014 - reportedly grew apart over their priorities, with Brad said to be more focused on his movie career.

A source told DailyMail.com: "Angelina wasn’t into wasting time on frivolous things. Brad felt it was important for them to live life to the fullest, but as the months passed, Angelina seemed to withdraw, making the children and her work with the UN a priority.

"Angelina wasn’t really interested in doing any project that took her away from the children and home, but Brad was still wanting the wanderlust, adventure-filled life that first attracted him to her.

"Angelina did not approve of this movie [World War Z 2] because she knew it would once again take him away from her and the family when she felt they needed to spend more time together."