There is a "succeeding permanent tooth" underneath the root of a deciduous tooth. When the permanent tooth erupts, the root of the deciduous tooth will resorb and the tooth will become loose and will eventually fall off. This natural phenomenon occurs between the age of six and thirteen.
When a person is suffering from severe gum disease, the dental alveolar bone will recede and the teeth will lose support and become loose.
When severe tooth decay or gum disease causes inflammation of the tissues around the tooth, or cause an abscess, the tooth will become loose.
After traumatic injury to a tooth, the tooth supporting tissues may be damaged and the tooth involved will become loose.