1998 Plymouth Breeze oil leaking

1998 Plymouth Breeze mileage: 75,525. I was finding oil leaking in my driveway. It continued to get worse. I brought the car to the dealer and he said it was a head gasket problem. What would cause this on such a new car? Was it something I did? It is covered under warranty but how much would the cost have been to fix it? Thank you in advance.