And now we know why. He's engaged to the show's makeup artist, Mezghan Hussainy, a source who knows the couple tells PEOPLE.
Hussainy, 36, has been sporting a large diamond ring on her left hand lately, reigniting speculation that the fickle judge had finally found his match.
Cowell, 50, and Hussainy "are very suited," Cowell's U.K. rep Max Clifford told PEOPLE last week. "She is someone who is independent and says what she thinks and feels, and he likes that. He's probably the happiest I have ever known him in the nine years I've been working with him."
Added Clifford, "They have known each other for years. They've actually been going out together for the last year. She knows him; she's been around him. So, she knows the person he is, and he knows the person she is. That's why it works so well."