Despite what Tonks thinks the dog is nothing to do with Charlie. Well he learns to love it in about six seconds, but he hadn't been the one to introduce it to their garden. Not that he minds; in fact a new friend is enough to make him forget that he's just lost his lie in by having a shoe thrown at his head.
Tonks has work, which means Charlie is in charge of the dog, which means that Charlie gets to have first go at bonding and also gets to name him.
The dog whines when Tonks leaves and avoids Charlie for over four hours despite his best efforts, and that leads to an unpleasant conclusion: whoever the animal previously lived with had hurt him.
And hurt him badly, Charlie eventually gets close enough to find out.