Here is a study about smiles and how they relate to marriage success.  The heat of the meat is that the more you smiled in old pictures, the more likely you are to have a successful marriage.  Some of the suggestions proposed by the researcheres seem kinda silly to me, like "someone who smiles when the photographer tells him to might be more subservient which can make a marriage easier to maintain."

I'm just thinking to myself that people who smile  more are more likely to have a keen grip on their lives.  I smiled a lot, and that is because I felt in control of what happened to me.  When you are in control of your life, you spend a lot of time making your own reality.  Practice at influencing your environment translates into practice at making things work for you, up to and including relationships.  There ya go.

That being said, I can't remember too many pictures of my dad smiling, and I always took that for a characteristic of a Marine who was always just ready to kill someone.