Matt Damon is candid about his enduring bond with Ben Affleck and how they've helped each other cope with fame. In the latest issue of PEOPLE, Damon, who stars alongside Affleck in the new movie "Air," discusses how their friendship has been a source of support since they shot to stardom with their 1997 drama "Good Will Hunting."

Damon, 52, describes their friendship as "everything to me" and believes his life would be "unimaginably harder" without Affleck, 50, his co-writer and co-star in the film. "It's been really helpful to check in with Ben throughout this whole surreal process over the last 25 years. I think the shock of becoming famous is so... I mean, that messed me up for a couple years," says Damon.

He continues, "Because the whole world just starts to treat you differently. And so your subjective experience changes. It's like somebody rewrites your code in the Matrix, but just your code. It's like, 'Oh, my whole experience has been altered in this really overwhelming way.' And so to have somebody who I've known my whole life who I can be like, 'Hey man, is this happening to you? Are you…?' And to check in with him, it was really, really helpful. Stabilizing."

Affleck agrees that their relationship is unique and invaluable. "Having Matt has meant the world," says Affleck. "It's not just Matt. It's Matt's dad who I adore and love enormously. He passed away recently. And his mom and stepdad, his brother. His kids, who just came to the premiere and made me want to burst into tears because they're so beautiful and grown up and amazing. You get to be part of somebody's whole life in that way. It's been an enormous gift."

Affleck and Damon's friendship dates back to their childhood in Cambridge, Massachusetts. As teenagers, they pursued acting together, eventually co-starring in the 1992 film "School Ties" and co-writing "Good Will Hunting," for which they won the 1998 Academy Award for Best Original Screenplay.

Their latest collaboration, "Air," portrays the story of Nike co-founder Phil Knight (Affleck) and executives Howard White (Chris Tucker) and Sonny Vaccaro (Damon) as they gamble on the company's future by persuading NBA rookie Michael Jordan to partner with them. "Air" is now playing in theaters.