Why do girls have their buttons on the right side of their shirt and boys on the left?

I'm guessing you're facing the shirt, because when I put my shirts on, the buttons are on the right. However, there are many short answers to this question. The first, because most people are right-handed. Hundreds of years ago, women were dressed by their slave/servant/handmaiden/lady-in-waiting. Men usually dressed themselves. And because most people are right-handed, it's easier to button a garment if the button is on the right. Thus another inane tradition was born.

The second, because the buttons on a man's shirt are on the left (when looking at it) so when it is put on it is on they are on the right-hand side. And since it is easier to work the buttons with your dormant hand, and 90% of people are right-handed they put the buttons on that side. So Way Back When in the "olden days" when women from money and/or nobility had chamber maids that would dress them, to make it (in general) easier for the maids, they reversed the buttons so that the buttons would be on the maids right side.

The third, because the placement of man's and woman's shirt buttons, left or right goes back to where the buttons were originally placed on the garment, front or back. Man's clothing always buttoned in the front and the buttons were set up to give a right handed man the easiest way to button his shirt. Woman's buttons were originally placed in the back. They therefore had to have someone button their buttons for them. In order to make it easier for the person buttoning up the woman, the buttons had to be placed on the left instead of the right. By the time woman's buttons were place in the front of the garment, woman's clothing makers were already accustomed to placing the buttons on the left hand side.

Why are girls shirts buttoned opposite of man's shirts. the answer dates back to the Victorian age. Dresses and outer wear for men were very stylish but still useful... Women on the other hand wore LARGE hoop dresses and corsets. These cloths were put on by there handmaidens or friends and relatives. The corset that was so common at the time made it impossible to finish dressing after it was installed. SO the buttons were reversed so that they would be correct for the person buttoning the clothes. This is also the reason fainting couches were created CORSETS.

There is really no practical reason anymore except it makes it easier to tell a woman's shirt from a man's.