It all really comes down to einsteins equation E=mc2. What the equation states is that energy and mass are the same thing just in a different state. So when you increase energy you also increase the mass and when you increase the mass you increase the energy.... So, say you have a 100 pound man and he is stationary, but then the man starts moving which now adds kinetic energy (energy is energy no matter what kind). Once the man started to move his energy increased which led to an increase of mass.... Now if the man was moving at 100km/h the increase in mass would be something like .000000001g, its literally nothing, right? But say if that same 100ib man was moving at close to the speed of light his mass would be much greater because he has so much kinetic energy. And what happens when something gets more massive?...... It slows down. Every atom in your body would get heavier so would all your neurons in your brain so even your thinking would slow down and you wouldn't be able to feel the effects of time dilation because everything would seem that it was moving at normal speed when in reality it has been slowed down substantially just like you.....,