In some cases, a person has trouble creating sufficient saliva, a condition known as "xerostomia." When saliva is insufficient, not only is there less neutralization of acid, but also food goes dwown the esophagus very slowly, and consequently the esophagus is not protected adequately. Diminished saliva often stems from an underlying illness, such as Sicca syndrome. But there are other causes; for example, some medications may greatly reduce saliva. Radiation treatments for cancer patients will also reduce the volume of saliva. And sometimes elderly people have problems producing sufficient quantities of saliva.
It's also true that some patients with GERD have an excessive amount of saliva. Too much saliva doesn't rule out GERD.