Constant pressure from chewing, grinding, or clenching can cause dental fillings to wear away, chip, or crack. When the seal between the tooth enamel and the filling breaks down, food particles and decay-causing bacteria can work their way under it, and you run the risk of additional decay in that tooth.
New fillings that fall out are probably the result of improper or contaminated preparation or from a break caused by a bite or chewing. Older fillings are generally lost due to decay or fracturing of the remaining tooth.