Yep, this was the most difficult thing to grasp when I started learning this stuff. I thought that somehow the body knows to slow things down when it is in moti

Yep, this was the most difficult thing to grasp when I started learning this stuff. I thought that somehow the body knows to slow things down when it is in motion. But when you think through it, it is easy. Its all in the reference frames. Imagine yourself going from Earth to Alpha Centauri in a constant velocity. In SR terms, this means you are in motion with respect to earth. Now imagine there is nothing else in the universe except you. Take away Earth, Alpha Centauri, all stars, galaxies, etc. Nothing but infinite empty space. Imagine yourself doing the same movement (I know it is a stretch and I am butchering physics here, since I asked to take away your source and destination, but you can form a mental picture). Now, in SR terms, I cannot say anything about your motion. You are neither moving nor stationary and it is pointless to say anything. But based on old Newtonian model, you can be considered "moving" in this big empty room of Universe (which later became theorized into Ether). When you grasp this and understand that motion is relative, also understand that time is relative. There is no giant universal clock which dictates what time it is for this giant room (which is again what Newton imagined). Einstein simply said there is no such thing (aka special reference frame). Once this is grasped, other concepts like time dilation will flow from it.