High power LEDs need good heat sinking if they are to have a lifetime close to that given in the datasheets. The first question might be why do high efficiency LEDs get hot? It seems counterintuitive that a LED with a lumen efficiency of around 50 lumens per Watt needs more careful thermal design than, say, a floodlight with a fraction of its efficiency.
Source: EEweb Articles
Link: Thermal Consideration