There's a reason why troops and cops tend to sit against walls watching all entrances, even in a social setting like a restaurant. There's a reason why even years after you leave the service, you might be at a black tie event or a children's party and you're still assessing the threat level of every man, woman, and child in the room. There's a reason why most of us usually have some sort of weapon on as at all times, even when it's "unnecessary" and there's a reason why we're leery of people that either won't look us in the eye or stare a little too intently. We know what people are capable of when they want to do evil. In God We Trust, Everyone else keep your hands where I can see them.