The most common cause of tooth loss in adults is due to poor dental hygiene such as not flossing, not using mouthwash and not brushing your teeth often enough.
When you do not take care of your teeth you get bacteria building up in your mouth which then leads to tooth decay and gum disease.
When you have gum disease and tooth decay and do not treat the gum disease or tooth decay in time then your teeth begin falling out.
So it's important to brush your teeth after every meal or at least twice per day and every other day or so use some mouthwash such as Listerine to clean your teeth.
You can also gargle with apple cider vinegar which kills bacteria in your mouth and prevents gum disease.