Well, actually, bubbles are not always perfectly round all the time, as you probably have noticed if you've ever used one of those toy thingies to blow soap bubbles. But bubbles want to be spherical, and if you blow one that's more cigar-shaped initially, it struggles to reshape itself. That's because bubbles basically are thin layers of liquid whose molecules stick together because they are attracted to one another, a phenomenon called cohesion [source: USGS]. This creates what we think of as surface tension -- that is, a barrier that resists objects trying to move through it [source: USGS]. Inside the layer, air molecules that are trapped can't get out, even though they're pushing against the water. But that's not the only force acting on that layer. On the outside, more air is pushing inward at them. The most efficient way for the liquid layer to resist those forces is to assume the most compact shape, which happens to be a sphere, in terms of ratio of volume to surface area [source: Popular Science].
Interestingly, scientists have figured out ways to make bubbles that aren't round, so they can study the geometry of the surfaces. They're able to create bubbles that are cubical and even rectangular, by suspending a thin layer of liquid on a wire frame that that is molded into the desired shape [source: NEWTON].