The most likely cause of toothache when eating sugar is tooth decay. Dental caries is the process of chronic progressive destruction of dental hard tissues caused by bacteria, which mainly occurs when inorganic demineralization and organic decomposition of dental hard tissues occur, and finally causes defects to form cavities. With the further development of the lesion, the scope of decay will become larger and larger, the infected tooth tissue will be closer and closer to the pulp cavity, and the symptoms of pulpitis will gradually come out, such as eating sweet food, or eating too cold or too hot food, will produce a transient irritating pain. In this case, a prompt visit to the hospital is recommended for treatment to soothe the pulp and to remove decayed tooth tissue for permanent filling. If the tooth is painful for a longer period of time, it is likely that the decayed tooth tissue has infected the pulp, in which case it is necessary to perform endodontic treatment on the affected tooth to remove the pulp completely and fill it permanently to restore the anatomical shape and function of the tooth.