The constancy of the speed of light is one of the main postulates of
special relativity. According to this postulate, the speed of light in
vacuum is the same in any inertial frame. In addition, special
relativity also requires that the speed of light is the maximum speed
of objects in vacuum. This may be the most mysterious and bewildering
part of special relativity. Then why does there exist a maximum speed?
And why is the maximum speed or the speed of light constant? Is there a

deeper reason?

Since speed is essentially the ratio of space interval and time
interval, it is a natural conjecture that the maximum and constancy of
the speed of light may result from some undiscovered properties of
space and time. Here I will argue that the discreteness of space and
time may indeed result in the maximum and constancy of the speed of
light. More on http://www.quantummotion.org/