Fleas can make them scratch and chew at themselves, but so can ticks, dry skin, poor diet (dry skin), and allergies to something in their environment. Flea washes, treatments, and anti-pest "flea collars" have chemicals that cause itchy rashes on some canines. There are also skin diseases such as mange, fungus rash, and some skin worms (especially around the rectum) that can cause them to scratch, chew, or lick excessively. When in doubt, and if you can afford it, take them to a vet for evaluation if a pink or red rash, bald spots/falling out hair, rough/dry/"scaly" skin, bloody rashs or sores, or little white or black "pods" at the base of individual hairs are observed (eggs of parasites). Some breeds need to have the fur pulled out of their ears regularly and their ear channels cleaned regularly to prevent infections of the ears that can be quite painful for them and cause excessive scratching at the ears or rubbing them against the floor/ground/objects. I hope this helps.
I just had another less complex response come to mind... :)
They are hungry and eating, their teeth need work on a bone/shoe/table leg/baseboard, and they itch. :)
Indeed all the above especially dry skin.
Yes teeth need work not only in canines but humans.
It tis indeed a complicated matter and this lady concedes to the Noble Gentleman his indepth answer to a not so easy question.