Why Bubbles are spherical?

Hint: Bubbles are small liquid films which enclose air or any other type of gas in it. They consist of liquid particles, so thus have the same tendencies as water. Water has the property of surface tension, which makes it show the tendency to decrease surface area, in order to reduce surface tension. Thus a bubble does the same.

Complete step-by-step answer:
Surface Tension is the tendency or property of liquid surfaces to contract into the minimum possible surface area. Due to this tendency known as surface tension, the molecules of bubbles try to acquire the minimum surface area by making groupings and the tightest possible surface area that can be achieved by a collection of particles is only by packing together to form a sphere.
A sphere has the minimum surface area of all shapes that enclose a given volume. So the surface tension tends to minimize the surface area and to compensate it the molecules group together to form the best possible shape for the situation, which is a sphere.
Thus the bubbles are spherical in shape.
Additional Information:
i) Bubbles have gas (air) trapped into it, and the free molecules of gas tend to free itself by hitting the outside walls of a bubble causing outwards pushing pressure, this is compensated by the inward attractive force; surface tension, being equal to the outward pushing force of the gas.
ii) The bubble would not shrink or expand because of the trapped air in it making a balance with the surface tension forces.
iii) Of all the shapes including pyramids, cubes, etc. The only sphere has the smallest surface area.
iv) A bubble can only be formed by two substances; gases and liquids forming a globule.
v) Bubbles may remain intact even after reaching the surface of an immersive substance because of an effect known as Marangoni’s Effect.
Note: Surface tension is a major attractive force in nature. It is the reason why insects, usually denser than water are able to float or slide on it. The SI unit of surface tension is Newton per meter or $N{m^{ - 1}}$.