I wouldn't consider this a problem. If you are smiling automatically, then you are subconsciously responding to their aura. There are people that I run across that will naturally make me smile when I see them because their attitude - the way they carry themselves - the way they smile and respond to other people - makes it so easy to respond in kind. It could be someone I don't even know, but there's something about that person that just makes me smile.
Consider yourself lucky. Smile and nod and move on with your day.