Ben Affleck "Not Happy" With Jennifer Lopez's "Endless Need For Attention."

The Greatest Love Story Never Told is a documentary that follows Jennifer Lopez and Ben Affleck's 20-year romance.

She may have paid $20 million of her own money for the film, but it may wind up costing her much more.

The actor, 51, will openly back his wife's passion project.

But in private, he doesn't understand why the singer, 54, wants to make their private lives the center of attention, an exclusive source tells Life & Style.

The insider said, "Ben has become quite skilled at holding his tongue, but it's known that he's not happy at all about J. Lo's endless need for attention."

In the document, the Oscar winner acknowledges as much. Ben says, "I've always felt that things that are private are sacred and special because they're private, in part."

In the movie, J. Lo even permits a group of songwriters to draw inspiration for her new album.

"Ben loves J. Lo dearly and he knew what he was getting into, to some extent."

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