Vietnam veteran Artie Guerrero now has Multiple Sclerosis. If he needs an assist, Sierra can pull him in his wheelchair. Sierra's a dog so starved as a puppy that she was given up for dead.
"She's a survivor, and I'm a survivor," Guerrero said.
CBS News correspondent Barry Petersen reports Denver's Freedom Service Dogs is one of the few in the nation using only rescued dogs.
"There is no reason to breed dogs when there are so many good dogs in shelters who have been beaten and abused," said Sharan Wilson, Director of Freedom Service Dogs.
Training begins with teaching the dogs that most humans can be friendly. Then, they master special tasks.
Donations pay the $20,000 for the nine months of training. So far more than 150 dogs have graduated and been placed. About 50 people are on the waiting list hoping for a companion. If a dog washes out, it gets adopted out. No dog is put to sleep.
It's not just how the dog helps with daily life - it's when the dog and the person come together - both lives begin to change.
Air Force veteran Suzanne Quinn has post traumatic stress disorder so severe that for six years, she wouldn't leave her house. Then Cabo arrived four months ago trained for special commands.
"She is restoring my sense of safety, my sense of security," Quinn said.
"Cabo came into you life, and what happens," Petersen asked.
"The world just opened up," Quinn said. "Doors I thought were slammed shut started to open again."
Remember Marley? He made it into the program and one day his devotion to serve may be repaid, with love.