"They seem to be really happy together," Stiller, 45, told Us Weekly at the Tower Heist premiere in NYC Monday. "They're both great people. I love them both."
Though Theroux, 40, has been shacking up at Aniston's recently renovated NYC penthouse, their future together remains uncertain.
"I've been married once and I don't know if I'll get married again," the 42-year-old actress added. "I can tell you that as of this very moment, as of today, I have no plan to get married."