The majority of the time when a dog acts like this is because their scared, not aggression. Once they realize your not going to hurt them, they flip a switch just like this pup did.

Dogs and cats can seem to react aggressive or angry, but most of the time in these kind of situations, they are scared or even terrified. It indeed helps to give them some time and try some affection and slow, soft talking. Not all dogs or cats accept their situation, but always be patient.