The thing that causes most hot water heaters to fail is corrosion of the tank. Water heaters contain a long magnesium rod (called the anode rod) that keeps rust from forming in the tank. It keeps the rust from forming by acting as a sacrificial element -- the magnesium bar rusts and dissolves so that no rust forms on the tank itself. This works great. Right up until the point the magnesium bar completely rusts away. At that point, the tank starts forming rust and it's just a matter of time before your hot water heater goes.

The magnesium anode rod in your hot water heater is only designed to last for four years or so. So to extend the life of your hot water heater, you need to replace this rod before it is completely gone. If you replace the rod every two to three years, your hot water heater should last for many years.