Please don't get annoyed with my question, I just want to know why....

This is part of universal standards of infection control. U will also notice that the dentist wears gloves, a uniform ( sometimes also an apron) and protective eye wear or glasses. Wearing a facemask and potentially protects both the dentist and the patient from micoorganisms in the air and aerosols, which are produced whilst he is working on you with his instruments ( especially when using high- speed handpieces, because they use water and are mixed with your saliva). A dentist or dental care professional may also have a cold or other air-borne diseases and may protect the patient as well. Universal precautions are vital to protect both the dentists and patients, from diseases and the dentists will not only wear facemasks when working on u, but also on every other patient.