I'm probably the guy everybody thinks is a jerk, but the (few) ones that get to know me usually change their opinion.
I mostly look bored. If I don't feel like smiling, I don't smile. Smile at me, and I will probably remain impassive. I hate when people expect me to act happy for no reason. But by my standards I'm very polite. I hate the ones that only greet people that have some kind of status.