The tissues inside your mouth are sensitive. Something as simple as over-zealous brushing can cause minor damage to the oral mucosa (the wet skin layer lining your mouth).
Other things which cause injury to the mouth, such as accidentally biting the inside of your cheek or tongue or a blow to the jaw, disrupt these delicate tissues and can lead to a traumatic mouth ulcer.
Another common cause of mouth ulcers is food that’s too hot, causing scalding to the mouth or tongue. Food that’s particularly acidic or spicy can also trigger them.
Crisp-lovers should watch out, too, as spiky, sharp foods are also known causes of mouth ulcers due to the micro-injuries they can inflict on the oral mucosa as we crunch them.