When it comes to those you don't know, there are usually two types of strangers:  Those you can't trust and those who don't trust you.  Those who are "out to get you" and those who think you might be out to get them.

The individual is like a nation, and like any other nation has certain secrets to protect.  Things that could discredt oneself, even if not all that bad but is prone to being misinterpreted in an adverse way.

You get too comfortable with someone you don't know that well and you divulge some deviant element of your past or of your character and suddenly they're like:  Is that so?
And you realize:  Did this person just entrap me?

That's the problem with people:  Either they're the type to pull a fast one on you or they're the kind who never seem to be able to trust you enough to be willing to give you a chance to prove yourself one way or the other.
The concept of the "normal person" is essentially the stuff of collective popular legend.   People are not those passive naive affected dorks one assumes most people to be.  They're proactive, dominating, sometimes aggressive, even spies, often conspirators.

The governments are not the ones who invented the concept of surveillance, although they're the ones who've perfected the practice.
But it was the paranoid average Joes and Janes who first initiated it.  In many a small town or suburbia setting outsiders, reclusives, and newcomers will find themselves frequently stalked by locals who circle around in their own vehicles while staring them down.

The trademark characteristic of any kind of paranoia is the way those who feel conspired against will themselves behave conspiringly toward the alleged perpetrators.