One of the most noticeable causes of dog scratching is fleas. Dog owners may see the fleas, see the flea dirt, or even get bitten by fleas themselves.
A severe flea infestation can cause hair loss and sores on the dog's skin due to scratching. Dogs may be allergic to flea saliva that can make these symptoms significantly worse.
In addition to flea saliva allergies, other types of dog allergies can cause dog scratching. Dogs can develop allergies to dog food, medication, vaccinations, or substances in their environments.