To establish useful warning systems for hurricanes, it is necessary to accurately predict both hurricane intensity and track. But although the forecasting of hurricane tracks has improved over the past 30 years, the factors that control the intensity of hurricanes are still poorly understood, leading to almost no reliability in forecasts of hurricane intensity evolution. Efforts to improve intensity forecasts have focused almost exclusively on characterizing the dynamical interactions between hurricanes and their atmospheric environment. Here I use a simple numerical model to demonstrate that, in most cases, the evolution of hurricane intensity depends mainly on three factors: the storm's initial intensity, the thermodynamic state of the atmosphere through which it moves, and the heat exchange with the upper layer of the ocean under the core of the hurricane. Such a limited number of controlling factors offers hope that, given an accurate forecast of a hurricane's track, its intensity can be reliably forecast using very simple models.