An Answer:

Been doing some reading.... Must warn you though, I've found the answer to your 'why?' question.... The answer is sadly, 'because'.

First of all, it's important to understand how laws in Physics are handled. If a law if found to predict perfectly a particular phenomenon, and there is no other law to refute this law, then it is considered to be true. These are called Hypotheses, or something. So as long as they are not disproved, they remain to be the authority on matters. For example, if some bugger finds out that a certain amount of a gas always had the same number of molecules, then he can state a law, which remains true until some other bugger finds out that some gas does not obey this. When this happens, there are two alternatives

i) The second bugger could formulate his own law which would group the older law, as well as this gas which does not obey the older one, but obeys the new law.

ii) He could say that this particular gas was the only exception to the law.


Now let's take the old relative velocity business... Even you should be able to understand this:

Assume two cars moving in the same direction with speeds of 60m/s and 40m/s. The chap in the slower car would measure the speed of the faster car as (60-40) = 20m/s. So the new velocity (v) recorded would be velocity of one car(u) - velocity of other car (w).

Thus:
v = u - w;


Now assume two cars approaching each other at 50m/s. Thus, one is moving at +50m/s, while the other at -50m/s(since it is in the opposite direction). Here the recorded velocity would now be:

v = u - w;
v = 50 - (-50);
v = 100;

So for two cars approaching each other,
v = u+w;

This was all pretty ok, since no one found anything wrong with it. All the experiments that took place proved this, and so there was no need for an alternative theory. That is, until Einstein threw a spanner into the works. Einstein's theory of Special Relativity had some basic ideas. One of them was that the speed of light does not vary with the motion of the observer. This was quite a bugger for people to understand, especially if you draw a parallel between cars and such. Imagine a car that remained at the same speed irrespective of the speed you went at!

Einstein realised this, and thus had two options. Drop the theory of Special Relativity or try to change the idea of adding velocities. Now Special Relativity described some phenomena with such accuracy that it could not be wrong. Thus the only other option would be to change the idea of velocity addition, which is exactly what he did.

He changed the formula (v = u+w) to:

   
v   =   u  +  w
        1 + uw
            c2
  

Difficult to understand? Sorry, couldn't really make it simpler. :) What Einstein did to the formula was to add a denominator. c is the speed of light here. Now assume you are moving at a speed of w m/s, and watching light move at c m/s. The velocity of light as observed by you would be:

v   =   c  +  w
        1 + cw
            c2
  

which can be written as:

  
v   =   c  +  w
         1 + w
             c
  

But c is so large, that (w/c) becomes very close to zero, and (c+w) is very close to c.
Thus the formula will now be:

v   =  c
       1

or v = c.

Thus the velocity of light you observe, will remain to be c, according to Einstein's equation!


If you haven't understood this, lets look at another case. Assume you are now moving at the speed of light (a fact, incidentally that is impossible, but nevertheless for the sake of argument).

Now if you are both moving in the same direction, classical mechanics (i.e, the old simple formula) states that you should observe the speed of light as:

v = u-w;
v = c-c;
v = 0;

i.e, the speed of light would be zero, but Einstein's theory doesn't support this. So, now let's try with Einstein's equation.

v   =  u  +  w
       1 + uw
           c2
v   =  c  +  c
        1 + c2
            c2
v  =  2c = c
      2  

Thus, Einstein's equation proves his theory, and thus explains why light always has a constant speed, irrespective of the frame of reference.

Another thing to remember is that this is not universally accepted. Some people still imagine that Einstein got it all wrong. Nevertheless, it is used as people found it to be a good enough answer. All we need now to bugger it all up is some other wise-guy who finds out that light's speed isn't constant or something. :)

Here's another way of looking at it. Somehow I like this method better, because it gets rid of all the messy and slightly suspicious limits and such. Though the other way is a little more straightforward:

Imagine a body moving at a speed... any speed. Let's call this speed 'k'. Now, k can be expressed in terms of c like this:

k = x * c;

Where x is any number. Thus, if a body is moving at say 100m/s, then x will be

 
k =      100     
c     300000000  

Now according to the equation:

 
v   =  c  +  k
        1 + ck
            c2  

Thus

 v   =  c  +  xc            [ since k = xc ?]
       1 + xc*c
            c2  

Thus

  v = c+cx
      1+x  

Thus

 v = c(1+x) = c;
       1+x  

Which implies that whatever speed you're going at, the speed of light remains constant...

You can measure the speed of light. What you can't do is go faster than it (electrons however, can be accelerated to 99% the speed of light). However, the point this equation makes is that whatever your speed is, light will always appear to be travelling at c (approximately 300000000m/s).