First you want to verify that those teeth actually have decay and that your other teeth do not have decay.
Then you should consider a cavity assessment. You have teeth; therefore you have something to decay. You could have more virulent bacteria (more cavity causing bacteria). Then consider your oral hygiene habits and whether there is any more you can do (brush once more a day, floss, fluoride mouthwash). Finally consider your diet (sugary foods and drinks, acidic foods and drinks - gatorade and the like wind up having a pH similar to your stomach acid). It is also important to consider when you consume these products; after your brush your teeth, right before bed. How long do they sit on your teeth.
-Not proper brushing
-Not using ADA-approved toothpaste
-Not enough saliva
1. Excessive consumption of sugar (candy) and acidic beverages
2. Consumption of carbohydrates or fruits rich in sugar
3. Poor oral hygiene
4. Little fluoride use
In order to ascertain as to what exactly is the reason in your case, you will need to be seen and discuss some history before arriving at a conclusion.