Why is the speed of light constant?

Question: Why is the speed of light constant?
- Keywords:
  constant,
  light,
  speed
Asked by charlie13 to David, Luna, Probash on 24 Mar 2011 in Categories: General.
- David Pyle answered on 24 Mar 2011:
  
  By definition, the speed of light in a vacuum is constant. This was first suggested by Einstein, just over 100 years ago, and has been verified by measurement.

Question: Why is the speed of light constant?