There are many reasons teeth become crowded. Most of the time crowded teeth pose no problem to the heath of a person. Sometime crowed teeth may be due to periodontal disease, which causes bone lost. Lost of tooth structure from decay can also cause teeth to be crowded. Other reasons can be poor tongue position, mouth breathing, biting objects like finger nail, etc
Modern orthodontic technique can correct many of these crowding problems. After teeth are aligned, flossing teeth can be easier and teeth keep clean longer.