I lead a very violent childhood. As a result, I am covered with old scars. The most dominant are the ones you cannot see. I suffered muscle damage at one time in a scuffle with someone way older than myself, and the long term effects of that still show.
When you smile, you have two dimples that show up. I suffered muscle trauma to one side, so there are no dimples there. I have to force a super smile to get anything to show up, and at that point, it just looks weird, and creepy. So I favor half smiles.