Can someone please tell me why my freezer continues to freeze up?

Two weeks ago the coils in my freezer froze up. I know this because I called an appliance repair serv. They told me they could fix it for $325. I of course said no and defrosted it myself with a blowdryer. It worked like new. However, today the same thing happened. The food in the fridge and freezer wasn't cold/frozen so once again I saw that the coils were frozen. I defrosted it with the blow dryer again but this time they froze up again almost immediately. What is going on?And does anyone know how I can fix it?