What is the rules behind it?
One theory says It has to do with warfare. All shirts had buttons on the left until the times when men fighting found it clumsy when drawing their swords and the hilt would get caught on the shirt (or coat) due to the way it buttoned. The buttons were moved so that the drawing of one's sword would not catch on the shirt (coat) thus making the action to be much quicker which can be a life saver in battle.
The answer Ladies' clothing always buttons right over left, men's clothing left over right. How has this come about? 'LADIES' were once dressed by maidservants; it is easier for a right-handed person to button another's clothing right-over-left. Men normally dressed themselves, and the opposite method is easier.
May be to know the shirt is tailored for male or female.
IFBB Official | State Head NPC Karnataka
1dBecause women had servants to dress them up and men dress themselves