Why does the seat of a swing stay parallel to ground when you left right on swing?

When you lift your right leg on a swing, your body's center of mass shifts slightly to the left. In order to balance this shift, the swing seat must also move slightly to the left. This causes the seat to tilt downwards on the left side and upwards on the right side, creating a torque that rotates the seat around its center of mass.

As the seat rotates, the tension in the swing chains increases. This increased tension provides a centripetal force that pulls the seat back towards the center of the swing. The combination of the torque and the centripetal force causes the seat to move in a circular arc, with the seat remaining parallel to the ground.