According to Livescience, hurricanes die when they lose their source of energy. Hurricanes require warm water to fuel their storm clouds, which is why they tend to form in the open ocean near the equator. When a hurricane moves into cooler waters or over land, it quickly loses strength and eventually dissipates.
The engine of a hurricane consists of warmth and moisture. When the air at sea level is hot and moist and the air above is considerably colder, the warm air rises and forms a low pressure region. As more air moves into the low pressure zone, it absorbs moisture and heat and rises. This carries energy into the upper level of the system as well as setting up a rotation. As long as a hurricane is over calm, warm seas, it will continue to grow in strength.
As soon as a hurricane moves over land and loses this energy source, it begins to expend more energy than it can take in. This is why hurricanes tend to weaken once they turn northward in the ocean, or once they make landfall. However, in some cases a hurricane's path may take it back over warm seas and allow it to strengthen once more if it hasn't completely dissipated.