Great question! There are usually several large hurricanes that affect the U.S., Mexico, and Caribbean each year. These huge storms also occur in the Pacific, where they are known as cyclones. So why do hurricanes happen so often? The general answer is that the ocean transfers a lot of heat to the air in the summer time. This heat is a form of energy and can cause violent storms. How much heat? I read one estimate that a hurricane can expend as much energy (heat is energy) as 10,000 nuclear bombs in its cycle (Grtozinger et al., 2007)! However this energy is expended over a huge area and over a long time, so it is not as powerful as a nuclear explosion. Hurricanes usually originate near the equator in the summer time, where ocean surface waters are very warm (usually above ~250 C or 77 F).

Warmer ocean waters mean more heat is transferred to the air above the ocean. As the air rises, it condenses and forms clouds and rain. Condensation of water releases heat; it is the exact opposite of putting heat into water to turn it to vapor. This heat that is released causes air to rise faster. When the hot air rises, it leaves behind a low pressure zone above the water beneath it. More air flows toward the low pressure, and the cycle keeps on going. All of the air moving from high to low pressure zones causes heavy winds (sometimes greater than 150 miles per hour), and the condensed water vapor causes heavy rains.

Reference
Grotzinger, J., Jordan, T., Press, F., Siever, R. (2007). Understanding Earth 5th edition. New York: W.H. Freeman and Company.