The skin of a bubble is composed of a thin layer of water molecules sandwiched between two layers of soap molecules. Water on its own has a high surface tension due to intermolecular forces causing molecules to pull on each other, trying to minimise the surface area and be as flat as possible.
Soap reduces this surface tension, however the effect of surface tension is still present in a bubble which causes the bubble ‘film’ to be stretched. A sphere is formed as it is the shape with the least amount of surface area for its volume.
Answered by Rik Sargent