Facial Expressions

I've been paying attention to myself a lot lately.  Not in a "taking care of me" way, but rather in a noticing things about me way.

What I've noticed the most, is my lack of facial expression most of the time.  Maybe it's not really a lack, but more like a scowl.  I bunch up the muscles at the bridge of my nose like I'm worried about something, and the rest of my face is slack.  Even when I try to "lighten" my expression, all I can manage is a softening around the eyes that doesn't really reach any other part of my face.

What I notice even more is that I have this scowl right up until I start talking with someone about something that is interesting to me.  Then, I can feel my face soften, my eyes light, and my smile come more easily.  I FEEL the difference in my face, and in my body.  It's almost like my whole body is lighter.  I know there are studies that argue that smiles produce serotonin, which is a happy hormone, or some such.  

What I want to know is, is this something that happens with age, or am I really just cranky all the time?  I don't feel cranky, yet when I'm at home or working I sound cranky.  Is it my face affecting my attitude, or my attitude affecting my face?  How do I lighten up?  I'm way too serious for my own good!