The most common parasite that will make a dog scratch is the flea. Even with the current excellent flea preventatives on the market, a lot of dogs suffer from fleas because owners don't use the products. Even indoor dogs should be treated because they can pick up fleas in the backyard or in parks. Mites are also parasites that cause dogs to scratch. Parasites can cause an itch so intense that large areas of hair are lost from scratching.
Allergies are another frequent cause of scratching in dogs.
Fleas are again the most common culprit in many parts of the country. Most dogs with fleas will scratch to some extent, but a dog with flea allergy is miserable after only one flea bite. Environmental allergies are similar to those people have. However, people usually sneeze and get a running nose and eyes, and dogs usually itch. Food allergies, caused by one or more ingredients in the dog's diet, are less common but can be more severe.