Periodontal disease and poor oral hygiene:periodontal disease is an infection of the gums and bone caused by the buildup of plaque as a result of poor oral hygiene.Its earliest stage is known as gingivitis(swollen ,bleeding gums) and can progress to periodontitis,where bone is destroyed and teeth loosen.
Trauma or injury:Accidents,sports injuries,or falls can knock out or fracture a tooth beyond repair.
Medical conditions:Systemic diseases such as osteoporosis, diabetes and so on can loosen the tooth.
Teeth Grinding(bruxism):constant grinding can wear down or fracture teeth,making them more likely to be lost.