Why the speed of light is relative to everything? Einstein tells us that speed of light is relative to everything but what is the main logic behind this which explains why the speed of light is relative to everything?

"Logic" may not quite be the right word to use here. :)

As beings who exist at non-relativistic velocities, time to us seems to tick along at a constant rate. Therefore, we have a natural inclination to think about things as a function of time. It seems obvious to all of us, for example, that speed is simply defined by the distance an object travels during a constant unit of time. As an object speeds up or slows down, the distance it travels during that same amount of time will increase or decrease respectively. Using calculus, we can perform the calculation in the limit as the unit time interval goes to zero, so that we can compute instantaneous speed even if it's changing.

The idea of special relativity, however, is that time is NOT constant. It only seems perfectly constant to us because we move so incredibly slow that relativity never really manifests itself. When speaking of traveling light waves, Einstein's idea is that the formula v=d/t (velocity = distance/time) still applies, but it is not time that moves along constantly. It is, rather, the speed of light which remains constant. Time and distance are the things which actually change (and they do so in proportion to each other, so that the speed of light always appears to be the same relative to any frame of reference, regardless of how fast the observer is moving).

I know that might not be the best of explanations. As I said at the beginning, it's hard apply logic to something which is so non-intuitive.