Since this is the "Space Science Board" I will point you to the scientific explanations.

One is from this web site:
http://www.wisegeek.com/what-is-the-highest-possible-temperature.htm

Under Einstein's theory of relativity, an object accelerated close to the speed of light gains a tremendous amount of mass. That is why no amount of energy can suffice to accelerate any object, even an elementary particle, to the speed of light - it becomes infinitely massive at the limit. If a particle is accelerated to a certain velocity near that of light, it gains enough mass to collapse into a black hole, making it impossible for observers to make statements about its velocity

More info here.

I'll explain my view on the Off Topic Board when I have more time.