Because the Captain sits on the left and gets to perve out the window at the good looking ones as they board. Has always been thus. Also, these days he is the one who gets the sack if the airplane hits the airbridge. Seriously, there were some DC3s built way back that had the pax door on the RHS - something to do with the way that that particular airline had sited its terminal buildings, or so I read somewhere. Musta been an enlightened airline that let the F/O park the ship? Geez I may be old, but not quite old enough to have seen the buildings, so can't confirm this story, tho' I have seen a DC3 or two with the door on the wrong side. Useless info....?