If your dentist has advised you that you need fillings, this is because you have decay in one or more of your teeth.
Dental decay is an infectious disease that destroys tooth structure. It occurs when the acids produced by bacteria dissolve the hard outer layer of a tooth (enamel), allowing the bacteria to penetrate the inner structures of the tooth. This results in a cavity being formed in the tooth.
Dental decay is caused by several factors, including diet (too much sugar and acids), oral hygiene (irregular brushing and flossing), and underlying medical conditions such as reflux.