Earlier quoted context omitted.
If you're left-handed, handwriting proceeds best by mirroring what the right hand would do (ie. write 'backwards'). It's far more natural. Alright, so others will need a mirror to read it - but that ought to be society's problem, not yours! ("Equal rights for the Sinisterous!") In a gradual shift toward ambidexterity, I've found that questioning society's insistence on 'right-ness' can be enlightening...
Presumably script that proceeds right to left (and even upwards) was originated by left handed scribes edit spellchecker : left handed scribes became left handed scones. The history of heavy baked goods in the development of human civilisation has been scandalously neglected by a blinkered academic community.
I'm not sure if that's really the cause. Furthermore in many ancient scripts the direction was quite unimportant, in some cases even reverting in each line (Boustrophedon).
Interesting take on direction of writing: http://shkrobius.livejournal.com/292987.html