It involves maths I can't be arsed to properly type here, as well as a graph or two, but the basic gist is that it's down to a combination of relatively and the lack of a resting mass.
Time dilates and space contracts due to relative motion and the degree depends on velocity. At the the speed of light, you reach the maximum amount that spacetime can be altered relative to an observer. Because mass can likened as an ability to resist change in velocity and as light has no mass, it is automatically produced at the maximum speed allowed by relativity. Gravitational waves seem to have a similar behaviour.
One can not help but imagine Microsoft as being ran by a thousand Homer Simpsons. -Obturator