It was "Jen who decided to move on," says the friend. "She is very fond of John and has thought he might be the one. She finds him funny, sexy and very talented. ... Plus he understands all the aspects of living in a fishbowl. But in the final analysis, she just got tired of his roving eye."

A vivid example can be seen in the new Us Weekly (hitting newsstands this week), which has photos of the singer frolicking in a hot tub in Cabo San Lucas, Mexico, on a trip without Aniston.

Apparently, Aniston gave Mayer a "three strikes and you're out" ultimatum - after learning about his "quickie" flings with a cocktail waitress and a promoter's assistant for his concert tour.

That was two strikes. Another dalliance, with a groupie, was the final straw for Aniston.

A longtime major Hollywood studio exec - and good Aniston buddy - adds this: "Above everything else, Jennifer is looking for stability and loyalty in a relationship. She still is hurting from losing Brad [Pitt] to Angelina [Jolie] - even after all this time."

(Source: Chicago Sun-Times)