Sears kenmore fridge,10 years old, defrosting all the time?

My fridge keeps defrosting itself. In the past three days it has defrosted twice! The first time I noticed it because everything in the freezer was no longer frozen and the ice bucket had started melting, dripping water. I cleaned it up and set the freezer colder. About 15 hours later the freezer was back to its normal temperature and everything was frozen. Then two days later I wake up and the same thing has happened. I know it is a automatic defrosting model but this has never happened before. What is wrong?