"The relationship sort of hit Cameron out of nowhere," a source close to Diaz tells PEOPLE. "She's always been open-minded when it comes to men. She just wants to have something in common with them and find a personality that loves life and loves as hard as she does."
Diaz, 41, took her romance with Madden, 35, public Tuesday, walking around New York City hand-in-hand with the Good Charlotte guitarist after a Starbucks run.
The casually dressed couple, sporting sunglasses and smiles on the coffee outing, have been spotted together often over the last month. On Sunday, they grabbed a bite a chic downtown Manhattan restaurant ABC Kitchen, PEOPLE has confirmed.
As for how the stars could have connected, Diaz is friends with Nicole Richie, the wife of Madden's twin brother, Joel.
So is Diaz, who last dated Yankee Alex Rodriguez through 2011, looking to get serious?
"She's at a point in her life where she would love to settle down, but for now with Benji it's about being happy and taking things as they come," the pal says. "She's happy … really happy."
Reporting by JENNIFER GARCIA