You may not be able to clean both sides identically with your cleaning hand. You then tend to brush one side too hard. This can cause the gums to recede, leaving the tooth necks exposed. The reason for the pain is that the enamel, which protects against external irritants, does not completely cover the root. If the gums disappear, dentin is exposed. Dentin itself does not provide adequate protection against stimuli such as cold or heat. This results in pain caused by the nerve processes inside the tooth.