Find this book, it's really good/helpful and scientifically-based (but written by a science journalist so not a difficult read) instead of just trying to make left-handers feel less inferior or more superior or something:
http://www.amazon.com/Left-Hander-Sy.../dp/0679744681
We become left-handed from pregnancy or birth trauma or something I didn't quite understand having to do with testosterone across the placenta or something.
I imagine the Polar Bear is just hard-wired to be left dominant if it's true. They're branch offs from the Brown/Grizzly bear:
http://www.wunderground.com/climate/PolarBears.asp
Would be interested to know the Brown bear's handedness.