Why does an object move?

Newton said, it moves because something/another object forced it to move(here motion means changing the state, doesn't include moving smoothly forever).

Einstein said, it moves because it feels like it.

ZhuangZi said, because it wants to.

In Newton's theory, time and space are absolute, an object could move forever if there were no other objects. (Maybe he was right,but no one can live forever to test this theory).

In Einstein's theory, there is no absolute reference frame, time and space are not  absolute.

However, speed of light is absolute in Einstein's theory.
In other words,  speed  of light stays the same whatever reference frame is used.
 
ZhuangZi might have said, what the hell is the speed? I only knew one thing, there was no absolute reference frame. Time and space could be different if one chose different reference frames. More than that, speed of light could also be different if one observed the world differently.

To summarize, time(250ears) doesn't make relativity a revolution, the struggle between "absolute" is what makes it.

Although both ZhuangZi and Einstein talked about the relativity of time, Zhuangzi was farther way from Einstein than Newton had ever been.

After all these being stated, we should remember that we are not going to challenge Einstein. Newton and Zhuangzi should be put away, at least for a while, let's go back to the topic, speed of light.