The Answer is in the Einstein's famous equation E= mc2
According to the equation, there is an equivalence between energy and mass.
So,the energy which an object has due to its motion will add to its mass.
That is, it will make it harder to increase its speed.
So as an object approaches the speed of light, its mass rises even more quickly, so it takes more and more energy to speed it up further.
It can in fact never reach the speed of light as by then its mass would have become infinite and by the equivalence of mass and energy it would take infinite energy to achieve the speed of light.
Only light, or other waves that have no intrinsic mass, can move at the speed of the light.
Information credit- A brief history of time by Stephen Hawking