Both cats and dogs will occasionally scratch themselves, just like people do which is normal. However, constant scratching (and licking/chewing) or scratching to the point of losing hair or causing the skin to be red is not normal. Itchy skin can be caused by a wide variety of thing including but not limited to fleas, allergies to tree and grass pollens, dust mites, and food ingredients. Many allergies are seasonal and often, but not always worse in the spring and summer. Dogs and cats who scratch constantly and disrupt the protective layer of the skin will be more prone to infection.