ghostly higgs ...

There are particles which are massive and also there are particles which are massless. According to the theory of relativity the massless particles are the only particles who can gain the ultimate speed. the speed of light. like photons. they always travel at a speed c. but the massive particles dont. Now, They also travel but the speed is rather slower. As they get more massive, the speed is slower. the very question, is how do they get mass. what causes them being massive?