Consider two parallel wires, each carrying an identical current I in the same direction. The moving  charges create magnetic fields, and react to each other's magnetic fields, causing the wires to attract
each other. (Curl your right-hand fingers and thumb around until you're convinced that's true.) But now consider the same scene from a reference frame that is moving with the current. In that frame the  electrons are (on average) not moving, so they exert no net magnetic field. Do the wires still move  toward each other? Why or why not?

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