It's in a dog's nature to be territorial and predatory, and that can mean getting stroppy and aggressive. Here are some reasons why your dog may growl or give the occasional nip:
Dogs can't switch between mental states like we can. If you've trained your dog to be an aggressive guard dog then you can't expect him to clock off and patiently ferry toddlers around on his back. He will remain an aggressive guard dog.
A dog may bite if he's frightened or in pain.
Older dogs with bad hearing or eyesight startle easily and may nip in fear.
Some dogs have been bred to be more aggressive. Check with your vet if you're unsure.
Of course, if your dog is going over the top with it, you should keep him well fenced in at home, on a leash in public and give him some obedience training if you intend taking him around strangers.
Important: Never leave a child unsupervised with a dog. Even if the dog has been fine with kids before remember that dogs are pack animals. This means that one day he may notice that the child has grown more dominant and decide to challenge them for their place in the pack rankings.
