Smile! (PLEASE!)

Why is it that I can never get my kids to smile nicely for a picture? Oliver lives in this serious face 24/7. I can't tell if he is deep in thought or just annoyed that I won't leave him alone. I am lucky if I get one shot of Tavin before he runs off, and don't even think about telling him to smile for a picture, it can be pretty frightening...he lowers his chin to his chest, squints his eyes, and bares all of his teeth in what seems to be pain. Sometimes I can get "nice" pictures of them when they least expect it. (ex. looking off into the distance, clapping, playing with something, laughing at a friend, etc.) I just don't know why smiles have to be so hard to come by! Hopefully, it will make me laugh when I look back on it.