Why?  Because Courteney Cox has ditched her longtime BFF Jennifer Aniston!

"It seems like Courteney and Jen’s close friendship has chilled in recent months, and it comes down to their lifestyles not gelling anymore," an insider told The ENQUIRER.

"Jen is almost a workaholic who spends her free time dating and hunting for Mr. Right, while Courteney is happily married and busy juggling work and motherhood."

Courteney’s husband David Arquette "can’t be thrilled with the idea of Courteney developing a roving eye as she cruises parties and restaurants as Jen’s wing girl."

So Courteney and David – whose daughter Coco is 5 – are spending more time with couples who share their interests, like kids’ play dates and family dinners, said the source.

That includes actress Laura Dern and rocker-husband Ben Harper, and Spider-Man Tobey Maguire and jewelry designer-wife Jen Meyer, as well as single mom Sheryl Crow, said the source.

Jennifer was noticeably absent from Courteney’s 45th birthday celebration at Nobu in L.A. on June 15.

"Courteney and Jen will always be friends," said the source, "but right now they’ve drifted apart because they’re at different stages in their lives, with different priorities."