When current passes through a wire then a magnetic field is formed. Therefore, when two wires carry current in the same direction then both the wires with have respective magnetic fields in the same direction and their total magnetic field will be large.
But when current between two wires flow in opposite direction then the magnetic field produced will also be in opposite direction. Therefore, both the magnetic fields cancel each other out. Thus, total magnetic field will be small.
As a result, wires which carry current in the opposite direction repel each other.
