Why are bubbles round?
The short answer
Soap-bubble film has surface tension, which tries to reduce the film's surface area. For a given amount of trapped air, a sphere has the smallest surface area, so a free bubble becomes nearly round.
What is surface tension?
Molecules at a liquid's surface pull on one another. The surface therefore behaves a little like a thin elastic skin that tries to shrink. Soap helps water form a flexible film that can hold air for a while.
When the shape changes
A bubble can be distorted by the loop, wind, gravity, or neighboring bubbles. Even then, a small free bubble is strongly influenced by surface tension and tends to return toward a spherical shape.