Bubbles are formed due to the surface tension of water. This tension is the mutual traction force that exists inside the object and is perpendicular to the contact surface of two adjacent parts when the object is subject to tension. The attraction between water molecules on the water surface is stronger than that between water molecules and air. These water molecules are like being stuck together.

However, if the water molecules are excessively bonded together, bubbles are not easy to form. Soap "breaks" the surface tension of water. It reduces the surface tension to only 1/3 of that under normal conditions. After the bubble is formed, a large surface tension is required to make the bubble bigger.