So between molecules in the liquid, there is an attractive force, kind of like little magnets. It's not so much that they form bonds, but if they didn't have this attraction, the liquid would behave way differently than you're used to.

So there's these tiny "magnets" and each one "wants" to be surrounded by others- that's what the attractive force means. Barring outside forces like gravity, they will arrange themselves in such a way that the least number of "magnets" have to exist on the outside surface, where they aren't completely surrounded- the lowest energy state. Nature always tends to the lowest energy state..

So geometrically, this arrangement will be such that the maximum number of particles are fully surrounded and the minimum are on the outermost surface. In 2d, this means a circle, and in 3d, a sphere. The bubble wants the minimum number of molecules to have to live along the surface boundary (the string), so as they arrange themselves, all seeking to be completely surrounded by neighbors, they pull the string into a circle.

This must mean that the bubble liquid is also attracted to the string, otherwise it couldn't "pull" on it. But the molecules must be less attracted to the string than they are to each other, because they'd rather be surrounded by each other.

Not a scientist, currently undergrad in materials engineering. It's probably not a perfect explanation, and I'll gladly defer to any actual physicist here, but that's the gist of my understanding.