Why can't massless particle exceed speed of light? Why massless particle can't exceed speed of light? 
 A: It's just what it means for a particle to be massless. For a general motion of a particle in the vacuum, the rest mass $m_0$ satisfies 
$$m_0^2 c^4 = E^2-p^2 c^2$$
If the left hand side is zero, it follows that
$$E=|p|c$$
which means that the energy-momentum vector is null (light-like). In general, the speed may be calculated from the direction of the energy-momentum vector as $v=pc^2/E$ and it equals $v=c$ if the formula above is satisfied. Massless particles are not only unable to move faster than light; they're unable to move slower than light, too. The massless particles must move exactly by the speed of light.
The general absence of any signals that move faster than light is required by the special theory of relativity; a signal or particle that would be moving faster than light would be equivalent (from a viewpoint of a different inertial system) to a signal or particle that moves backwards in time, which would lead to logical contradictions (killing grandfather before he met your grandmother etc.).
A: To be absolutely simple.... A particle has got a certain mass and acquires a certain dimension in space..
We CAN'T define a Particle with mass=0. 
