Gravity

In order for a swing to work you need gravity. If you don't have gravity, you can't swing. As you are swinging, gravity helps pull you back down to the earth in an arc to create the motion that a swing has.
At the maximum kinetic energy, the forces acting on the swing is the force of gravity downwards and the normal force from the swing seat on you. If we were to look at the force of gravity at the maximum potential energies we would have to add tension from the swing's ropes into the equation.