"A Dog's Purpose," the movie for anyone who's ever loved and lost a dog

“When you adopt a dog, you have a lot of very good days and one very bad day,” author W. Bruce Cameron said. 

That’s where “A Dog’s Purpose” comes in. 

“In the end, it’s really the story of unconditional true love that never dies,” Cameron told CBS News of his bestselling book, now a major motion picture. “It’s told through the eyes of a dog who is reincarnated, so that’s the connection. True love never dies because this dog -- who gives unconditional love -- never dies either.”

It’s a message to which anyone who’s ever loved and lost a dog will surely cling, one that speaks to the question: How can a person survive the loss of a pet to whom he or she is deeply connected? And fittingly, it all started when Cathryn Michon, the woman who is now Cameron’s wife and the co-screenwriter on the film, was mourning the loss of her beloved dog Ellie.

“She wasn’t my wife at the time, but I was like a shark in the water. I was kind of circling her,” Cameron said. “She had recently lost a dog, named Ellie; the same name as the German Shepherd in ‘A Dog’s Purpose,’ which is a total coincidence, of course. And one day, she turned to me and she said, ‘I can never have another dog. I just can’t do it.’ And that wasn’t going to work for me. I’ve had dogs since I was a boy.”

So Cameron set about changing Cathryn’s mind the best way he knew how -- by writing her a story.

“I never met the real Ellie,” he told CBS News. “She died shortly before I came on the scene, but from what I understand she was a purebred Doberman Pinscher who was very high-strung, very smart and utterly, utterly devoted to her person. So with the book, I wanted both to comfort Cathryn and to convince her that the number one thing her dog would want is for her to get another dog.”

In the end, that’s exactly what the couple did, adopting a little 24-pound “bruiser” named Tucker, who lives at Cameron’s feet these days whenever he’s writing. It’s also what Dennis Quaid’s character does in the film. And that’s precisely what ends up reuniting him with his beloved childhood dog, Bailey.

“I hope that people who see the film will be moved by the love that these animals have for us,” Cameron said. “We bred dogs to be with us. That’s their purpose. We’ve been breeding them for 30,000 years. The message of a dog’s purpose is love, and I want people to come out of the theater smiling because they have witnessed such unconditional love on the screen.”

Fans of the book can expect a somewhat trimmed-down version of the plot, but Cameron insisted the essence of the story remains.

“One of the first things that happened when my wife and I set out to write the screenplay for the film is we had to cut things because a movie cannot possibly be as long and in-depth as the book,” he said. “It’s never easy to adapt a book, especially as the author, because it’s as if you’re chopping off appendages. It really feels painful to decide what has to go.”

When the ink had dried, however, Cameron said the primary difference between the book and the film was that the film version simply did not have room for the all the original’s depth in terms of character backstories and side plots. 

“I think people will probably enjoy the movie better if they read at least one of the books first,” he noted. “I will say, though, that the tone of the novel is captured on the screen -- the emotion, the love of these animals for their people.” 

In fact, emotion may be the operative term here. This is a film that will undoubtedly invoke nostalgia, one that will speak to dog lovers on a very personal level and one that’s not likely to leave a dry eye in the house. For the author, though, one scene in particular stands out as the most emotional.

“There is a scene that made me cry, it was so moving to me,” Cameron recalled of the first time he viewed “A Dog’s Purpose” in full. “The dog, Buddy, is racing across a field. And we have a sense that we know where he’s going and why. And he’s bounding through these tall grasses with such delight, such joy, that it just moved me to tears to see him.”

“A Dog’s Purpose” opens in theaters on January 27, 2017.