Memory loss more common in people with blood type AB. Several research studies have pinpointed lifestyle changes individuals can make to prevent memory loss, such as keeping stress and blood sugars low, and not smoking.
A dog will shake its head under normal conditions for a few reason, such as when the dog has just gotten out of the water or to shake seeds or burrs from its ear. Even a dog hair can irritate the dog enough to make it shake its head. When your dog shakes it head, it is important to check for any foreign objects, allergic reaction and bites in its mouth, throat and ears if you can.
It certainly isn't always the case that a dog is fearful, cold or even excited about something. In more serious cases, shaking could be the sign of illness, including Distemper. But for a dog who seems to shake at random times and has done so from an early age it is much more likely to be 'something they just do'. You must forgive me for my frustration at not being able to get to the scientific bottom of answering the question of why do dogs shake.