Bubbles aren’t always round! Their chemical properties make them try to have the smallest possible surface area, which is usually a sphere. But if you have a look at this page (http://bubbles.org/html/questions/round.htm) you’ll see that bubbles can even be square.
I don’t know much about this, so my explanation might not be very good. But this is how I understand it:
Bubbles are formed by a ‘film’ around a pocket of air. The film is flexible and so it takes on whatever shape results in it being under the least pressure. Because of the forces acting on it from within and outside the bubble, the least pressure results from a spherical shape.
I hope that one of the other scientists can give you a better, more scientific answer though!
This question takes me back to my physical chemistry days! It is because of their surface tension. The air pressure inside a bubble is greater than the pressure outside, and the force that this pressure difference causes is exactly matched by the surface tension of the bubble. Any deviation from spherical, and these forces would be unbalanced, and the bubble would adjust itself until it becomes spherical again.