"Everyone talks about the joy of having kids -- blah, blah, blah," he said. "But I never knew how much I could love something until I looked in the faces of my children."
Pitt and his wife, Angelina Jolie, are parents to six children, three who are biological -- Shiloh, 9, and twins Knox and Vivienne, 7 -- and three who are adopted -- Maddox, 14, Pax, 11, and Zahara, 10.
With so many kids, one could expect chaos, and that is exactly how Pitt describes his family of eight -- in a good way, of course.
"[There's] a lot of love, a lot of fighting, a lot of refereeing; a lot of teeth-brushing and spilling…," he explained. "Chaos, total chaos. But so much fun."
The Springfield, Missouri native also tells the outlet that having a big family was always a dream of his, and if it were up to him, he'd have even more!
"Listen, Angie and I were aiming for a dozen, but we crapped out after six," he said with a laugh.
Jolie, 40, and Pitt, who met on the set of Mr. & Mrs. Smith in 2004, have always had a desire to do a "small, intimate" movie together in which the two could have the freedom of bringing their kids to set. By the Sea, a new film written, directed and starring Jolie, was the couple's perfect opportunity to turn that dream into reality.
Although Jolie has directed two other movies, this was the first time she had the job of directing her husband. "I need to know I'm in good hands [with a director], and I trust Angie with my life," he said. "I love her instincts. She's ferocious with a story and she's really decisive at her post – in command."
"She's my wife," he continued. "There's no dividing line when the camera's rolling or when it's not. We're having the same conversation when we're making breakfast, when we're setting the shot, when we're in the scene or out of the scene."
Pitt adds that the couple in the film is "in that stage in life that the old fairy-tale books don't explain -- when the romance has dulled, the banality of the everyday has become the constant."
"Certainly the attrition rate of Hollywood couples looms large," he continued. "I'm surprised how much our history -- Angie's and mine -- means to me. That we have this story together. That we know each other. That we watch each other getting older, through amazing moments, joys, pains."
Pitt says that he's "surprised" that he and Jolie really know each other and share such a special connection, seeming to beat the marriage odds. "Because you hear people talking about the old ball and chain, and people trying to recapture youth, as if that's the impulse -- but it's not the impulse, it's not the impulse at all," he explained "There are no books to tell you what year 12 [of marriage] is supposed to be like, and year 14 and year 23 -- no guidebooks."
Pitt says that he can thank his family for his career, too.
"I was a pretty good actor before, but definitely hit and miss," he admitted. "I think I became a really good actor. I'm sure a lot of that has to do with age and wisdom too. But I see an absolute shift from the day I started my family."
"They need you," he said of his kids. "So I think that made me less tense about things, less stressed. And very clear that if I'm going to be doing something that's going to take me away from my family, it better be something that I believe in, and that they're going to see and be proud of their dad."
By the Sea not only gave Pitt the chance to work alongside his family, but it also allowed him to gush over his wife in many interviews, including when he spoke with ET.
Earlier this month, ET caught up with the couple on the red carpet at AFI Fest in Hollywood, California, where they revealed what it was like working on set together.
"It was a good time," Pitt said. "We came out stronger on the other end. Just, making something together, and something she wrote, it was a real pleasure."
"She knows when I'm faking it better than anyone else, but, you know, the actor/director relationship is a very collaborative experience," he continued. "It's really no different, and actually a great pleasure, 'cause you've got to trust your director. Who do I trust more than my wife?"