Well, when we got mice, they felt kinda optional. So organically we used the left hand for them. I keep it like that now. There are way more buttons on the key

Well, when we got mice, they felt kinda optional. So organically we used the left hand for them. I keep it like that now. There are way more buttons on the keyboard - more work for the right hand. Default WASD gaming is also more solid: place the thumb of your right hand on the left Ctrl - you can do a healthy rotation around this point while all other fingers are always above some relevant keys. Mouse on the left is useful if someone is sitting alongside you (on your left) and wants to point something on the screen with your mouse.