Why do some people produce extra saliva in their mouth?

Body produces it. Excessive saliva can be caused by either an increase in your body's production of saliva or a decrease in your ability to swallow or keep saliva in your mouth.
Thick Saliva. Thick saliva can be caused by dehydration, post nasal drip, allergies, sinusitis, upper respiratory infections, diabetes, hypoglycemia and radiation therapy. Also foods, chemo and medications.