Its a good question....I alway thought it was strange that our non dominant hand is seemingly doing the more intricate job (fretting). Its kind of like baseball. I bat right handed, if I actually was right handed my non dominant (read weaker) hand would be on bottom of the bat where the power comes from in your swing...I always wondered if this gave me an advantage having a dominant hand in the power position.
As for why I chose to play instruments lefty. When I started as a child I grabbed an old banjo we had lying around and started messing with it, teaching myself little one string songs. It wasnt until a while later a family friend who played guitar told me I was playing lefty, and what it meant. Until that point it had never occurred to me, it was just the way I had naturally first grabbed that old banjo. I wasnt playing any chords, just picking out notes so I never noticed a difference...I like being a lefty, its like being in an exclusive little club. As far as not trying guitars in stores I have adapted and can play quite a bit lefty on a guitar strung righty.