The accepted reason is that in past times, ladies who could afford clothes with buttons on could afford a maid to help them dress, and it is easier to button another person's garments if they are button 'right to left'. A gentleman on the other hand would be handed his shirt by his valet, and button it himself. It is suggested that in the days when swords were worn, a 'left to right' buttoning reduced the chances of a sword being caught in clothing when it was drawn.