A bubble, like a balloon, is a very thin skin surrounding a volume of air. The rubber skin of the balloon is elastic and stretches when inflated. If you let the mouthpiece of the balloon go free, the rubber skin squeezes the air out of the balloon and it deflates as it flies around the room. The same thing happens if you start blowing a bubble and then stop. The liquid skin of the bubble is stretchy, somewhat like a piece of thin rubber, and like a balloon it pushes the air out of the bubble, leaving a flat circle of soap in the bubble wand. Unlike a sheet of rubber that when unstretched loses all tension, a bubble always has its "stretch" no matter how small the surface becomes. If you blow a bubble and close the opening by flipping the wand over, the tension in the bubble skin tries to shrink the bubble into a shape with the smallest possible surface area for the volume of air it contains. That shape happens to be a sphere.