Why are security guards often rude?

Whenever I go to a concert, sports event, etc, I notice that the security staff is usually very rude. Even politeness (smiling, saying thank you), is often met with a cold, emotionless face and no answer. I understand that their job is to keep people safe and not engage in futile chit chat, but their rudeness ruins the mood of the events. I was wondering if maybe they're instructed to act that way?

Why are security guards often rude?
Post Opinion