Why do hurricanes form over warm water?

The warm oceans act as a source of energy for hurricanes.

Hurricanes form as large low pressure systems in the ocean but also sometimes on land and then can evolve to tropical , then tropical depressions and other stage up to a hurricane.

But when they pass over warm ocean waters they really pick up energy. This is what happened when Hurricane Katerina passed over the warm waters of in the Gulf of Mexico - it picked up a lot of additional energy which lead to more destruction in New Orleans.