hehe
yes I do study physics...
Firstly one big part is the fact that the photon - a particle of 'light' is massless. All particles with mass cannot reach the speed of light. This is relativity.
It is basically because of regardless of whatever frame of reference, light can only travel at the speed of light, c (which is about 300000000 m/s)
If you travelled at 0.999c, you still have to see that light travelling at c away from you, yet a stationary observer still has to see the light travelling at 'c'
It is also due to time dilation, I think. The faster speed you travel at, the slower time 'feels'
If you go so fast, to you time will be like normal, but if you got on a spacecraft, went to say Pluto and back at a high speed (Very high) when you come back you would be a lot younger then the people you left behind...