They Call It Puppy Love
At the end of A Dog's Purpose, the dog's purpose is revealed to be the role of wingman for his human companion, Ethan. The book veers into Must Love Dogs romantic comedy territory as the dog follows home another dog who smells like Ethan's childhood sweetheart, Hannah. Once at Hannah's house, Buddy the dog finds his way in. She reads his collar, learns he belongs to her high school love, and the two reunite to live happily ever after.
But as life has a tendency to do, the book swings from comic to tragic territory. After a few happy years, Ethan experiences a brain aneurism and begins to die. Buddy knows that his purpose this time isn't to fetch Hannah. She's away, and that would be impossible. Instead, his job is remain Ethan's stalwart companion until the end.
Ethan hallucinates that Buddy is his childhood dog, Bailey, which is pretty much true. Buddy thinks, "I wanted to let him know that, yes, I was Bailey. I was his one and only dog" (32.34). So he fetches their favorite childhood toy one last time to convince Ethan he is the same dog. In his final moments, Ethan says, "I will miss you, doodle dog" (32.43), which was Bailey's nickname. In that moment, Bailey knows, "I had fulfilled my purpose" (32.48).
But a dog's work is never done. Bailey vows to support the family through the grieving period; Ethan would have wanted it that way. The book ends, so we're left assuming that this is the dog's last life in a series of epic adventures, but who knows? Maybe he was reincarnated as your dog.