An occupation appealing to those whose internal mental resources are sufficient to stave off the extreme boredom this job can present.
Security guards function as the efficient (if somewhat indifferent) immune system of organizations, corporations, and the like.
Their primary purpose is to prevent theft of property (within limits of course; if you're waving a gun around they're probably not going to argue with you, and rightly so all things considered), keep anyone who looks like they shouldn't be there out, and expel those who the suits have decided shouldn't be there any longer.
The security guards I've met all seem to be friendly, polite, quiet, and not entirely inflexible when it comes to the rules. A trait more people would do well to adopt.