A Dog's Purpose is told by a dog. The text isn't 300 pages of "bow wow," "yip yip," and butt sniffing. This dog has the vocabulary of a college philosophy student ruminating on death and rebirth. It's a unique way of telling the story, since the dog cannot "talk" to humans or other animals. While it may be possible that the other dogs in the book—and yes, maybe even a cat or two—have similar complex thoughts, we have no way of knowing what is going on inside their little animal heads.
Telling the story this way allows W. Bruce Cameron to explore both the philosophical meanings of life and the humor inherent in, say, trying to housebreak a dog when he doesn't understand human language. While the narration style may be in the realm of fantasy, with the dog having an advanced vocabulary at birth, each life the dog lives feels very real.