The square ones pop each other? :)
surface tension and the lack of another amount of friction in the away. Though if you blow one big enough, you will notice a gravitational sag in the downward hemisphere making the bubble appear pear shaped! lol
To prevent the dirt from hiding in the corners.
Soap itself is like a balloon that is not inflated. When air is inside of it, it becomes an inflated balloon, in the form of a bubble...the way I put that was really weird, but that's how it works lol.
Physics... and all that jive.