Landov; WireImage

Paris Hilton and Stavros Niarchos, who have been dating on and off since 2006, called it quits today, an insider tells Life & Style exclusively.

"Paris dumped Stavros," says the insider. "She's been getting tired of him for a while. She feels he's just too insecure and immature.”

As for Stavros’ reaction?

"Stavros was devastated,” says the insider. “He's really bummed out. He was crazy about Paris. Paris says she really cares about him as a friend, but she doesn't want to be in a relationship right now. She loves him, but she's not in love with him."

The insider says Paris and Stav have not been out together in months. In fact, he didn’t even attend her St Patrick's Day bash on March 17, where, a partygoer says, she got hot and heavy with Josh Henderson, one of Hollywood's most eligible bachelors and Ashlee Simpson’s ex.

"They were kissing on the couch like two giddy teenagers. It was really cute," says the insider.

The heiress met Josh, who stars on Desperate Housewives, on March 16 at Les Deux, says the insider, where they obviously hit it off.

“The chemistry between them was intense,” the insider says.

Adds a friend, "Paris is so much happier now that it's over."