"Pets know exactly how close to draw near, they are great readers of body language," said Becker. "If they're with their owner and the owner is having anxiety, they read that and draw close just the right amount at the perfect timing. That's something humans aren't capable of doing.
"Dogs are more dependable than death and taxes," added Becker, "especially when it comes to emotional rescues."
And it's not only the human who benefits from the relationship, said Becker, who told ABCNEWS.com that "working dogs are happy dogs," and often enjoy the attention they get when they're on the job.
While Campbell admits what a huge help Pax has been to his slow recovery, he said that, at first, the dog was more stress than he was stress reliever.
"When [Pax first arrived], having him here was very stressful," said Campbell, who explained that during Pax's first weeks, he would sometimes bark unexpectedly, which was difficult for Campbell to cope with, given his sensitivity to sudden noises.
"PTSD makes you not want to go out in public and makes you not want people to talk to you," said Campbell. "You just don't want people around you.
"And when you do go out he's a magnet for people," said Campbell, who added he is still uneasy talking to people. "People come up to you and want to pet him and talk to you. It is very uncomfortable for me.
"I think as time goes on, things will get better," said Campbell, who visits a psychotherapist twice a week, and is in constant contact with his doctors.
"He's my friend," said Campbell. "I call him 'my buddy.'"