Why wont your battery on your 2003 dodge ram 5.9L V8 hold a charge when it is off?

Possibly dead cell in battery which would mean the battery is defective and needs replacing. Also can be some light on the vehicle is on and pulling power from the battery. Can also be a relay is stuck closed. Check all lights such as under hood, trunk, glove box, dome, courtesy, and brake lights. Check p/w, p/l, p/s, fuel pump relays.