Why does the rotor rotate?

A motor rotor rotates due to an interaction of magnetic fields of the rotor and stator, where either may use permanent magnets, electromagnets or a combination of both to effect the interaction. This interaction is based on the principle of opposite attraction and like repulsion related to magnets. Either the rotor or stator creates an electromagnetic field and the opposite component tries to attract or repel the appropriate magnetic poles to lock position with the electromagnetic field. The magnetic field is "rotated" electrically by applying voltage to different winding within the motor in sequence and since the moving component always tries to lock position with the magnetic field, the rotor will constantly rotates and tries to align its magnetism.