Tension differs on both sides of a pulley due to friction, which affects the net torque. The tension from one rope attempts to rotate the pulley in one direction, while the other rope's tension works in the opposite direction. For the pulley to rotate at a constant speed, the net torque must be zero, meaning the torque difference from the two tensions must counteract the frictional torque. Friction is applied at the pulley shaft, opposing its rotation, leading to a non-zero tension difference in the ropes. Understanding this relationship clarifies why tensions cannot be equal when friction is present.