Why 2 year old battery won't hold charge?

It has a dead cell. I know it is only 2 years old but it happens especially if that battery was run completely down which shortens the life. I suggest you purchase a different brand as this brand was not very dependable. My answer is assuming nothing is on pulling power from the battery when the car is just sitting. Have the battery checked to make sure it is actually defective. Any auto parts store will check it for you. It has to be fully charged in order for it to be checked.