I think smiling makes people alot more approachable. I mean as far as I can remember, I've never actually spoken to anyone who never smiled at me.
I don't know why.
I just think that when people smile at you it's like they're saying "Hey I am nice come and talk to me". :D
And as for me, I'm not always smiling into thin air but I smile at people. Probably because of how I think it makes me look friendlier, not because I'm insecure!