Question

Why rest and motion are relative ?

Answer 1

well take an example,
when you are sitting in your room, what do you think are you at rest or in motion? 
actually in relation to the walls of the room you are at rest.
but the earth is constantly in motion, rotating on its axis and revolving around the sun. thus you are in motion in such case.
therefore you can't say whether a physical body is in rest or motion unless you relate it to any other physical body.