04/03/2012 at 07:00 PM EDT
A decade after Chris Klein found success in films like Election
and American Pie
, his family and friends knew the actor had a problem with alcohol
. But they weren't the only ones: His dog, German shepherd Chief, realized it too.
"He would get agitated that the alcohol was coming out, and I will be forever grateful to him for that," Klein tells PEOPLE.
"He began to recognize the behavior shift in me when [I drank] alcohol and his behavior would change. Anybody that has a meaningful relationship with a dog will understand that the disappointment that I saw in his eyes, [it was] insurmountable."
The 33-year-old star, who is reprising his role as Oz
in American Reunion
, believes the beloved dog he adopted in 2008 helped him realize his problem with addiction – which he sought help for
at Cirque Lodge in Utah after being busted for a second DUI
in 2010, with then-puppy Chief as a passenger in his car.
"I drank away anything and everything that was at home," he says of that life-changing night. "I had nothing to come home to. My beautiful, wonderful dog was taken care of, he didn't get hurt, he didn't even have to go to the pound that night. They allowed me to call a friend of mine in the area to pick up my dog."
Now 19 months sober
, Klein is thankful for the friends, family and mentors who helped him turn his life around – and for Chief, with whom he cherishes every moment.
"On a Sunday I don't wake up hung over," he says. "I sit in my dining room with the sun coming in through my garden. Then it's Chief and me for the day, jogging on the beach, hitting the hiking trails."