why is potential energy said to be relative.
Answers
When we refer to potential energy we often mean the energy due to an object's position in some sort of gravitation or chemical field.
Besides the zero of energy being arbitrary as others have already said, so are the components of fields. So for example an electron stationary in your reference frame produces an electric field so that there will be potential energy due to this field for any other charge around.
However if you now change your reference frame you will see the electron moving and so forming an electric current. This current in turn produces a magnetic field so that any moving charges see a magnetic field.
So by you moving yourself you have changed the potential energy that two charges had from electrostatic to electro+ magnetic.