The etiology of babies always having mouth ulcers is complex and unclear, but it may be related to infection, oral mucosal injury, genetics, food, nutritional deficiencies and other factors. 1. Infection: for example, bacterial and viral infections can be associated with the occurrence of oral ulcers. 2. Oral mucous membrane damage: hard food or too hot water can cause damage to the oral mucous membrane, inducing oral ulcers. 3. Heredity: recurrent oral ulcers have a certain genetic predisposition. 4. Food: oral ulcers are related to the consumption of spicy and other irritating foods. Some of them can also be caused by allergy to food. 5. Nutritional deficiencies: Vitamin B₁, B₂, zinc, iron and other elements of the deficiency can also cause mouth ulcers. It is recommended to consult a doctor in time to identify the cause and then treat it.