This is only tangentially related to a recent experience I had in game, but it got me thinking over my history in RPGs - both table top and otherwise - and the interactions I've had with numerous city, village, and town folk. See, in RPGs, you end up walking through towns and cities a whole heck of a lot, and while doing so, you meet a wondrous variety of people. People who are helpful, and generally only rude or suspicious when they have something to hide. A GM recently turned this on its head, everyone was suspicious of the party, but it wasn't because they had something to hide. Their reasoning was perfectly normal, and felt a lot more real to me when I thought about it. So why were they so suspicious?
We Didn't Belong
People have had a general distrust for outsiders for as long as there has been people it seems. Generally speaking we trust what we know, and distrust - if not outright fear and hate - what we don't know. This applies to people as much as it does to everything else. Look at some of the old movies, particularly westerns, but also movies from the 60s and 70s. There is a strong sense, at least somewhat, of the community, and the person passing through is generally tagged as such. The western common name of "Stranger" isn't really all that endearing, however polite the person who says it may be. It really only means one thing "You're not from here, and we don't know you."
Now, obviously people who live in big trading cities where people pass through all the time are going to be more accepting. But that doesn't mean they're going to be less on guard. Who are you more likely to help? The person you know, and have done business with before (assuming it went ok), or the total stranger? Whose word are you more likely to believe? Who are you more likely to trust protecting what you care about?
We Asked Strange Questions
Bad enough that most PCs who are passing through a town are strangers, but they also ask weird questions. Not just "who is that person?" or "Where is the inn?" but things like "When did the last girl disappear?" and "Why wasn't someone watching to make sure the girls didn't leave their rooms?". This nosiness into other people's business can just put people on edge. Especially when the questioning gets more direct, or even starts pointing suspicion at the people the examinee has known their whole life.
PCs do this all the time too, hell, it is how you get information to go on the quest and figure things out. Still, how would you feel if someone you've never seen before started asking questions, and then those questions started to point blame towards the guy you've known for years?
We Were Armed To The Teet
This is the last part I'm going to cover for today. When was the last time you saw a PC not armed in some way? No gun, sword, knife, bat, or other weapon? What about armor? The lives of PCs are often fraught with danger, and so the PCs are well armed, armored, and otherwise equipped. So now, not only are they strangers with weird questions, but they're strangers with weird questions carrying enough weaponry to level the town that they're standing in.
Weapons make people nervous, especially when they're present in areas where not many people have them. At the very least they make the local guard/police nervous. Seriously, you want a cop to start following you around - if not flat out arrest you - let them see that you're the only person with a baseball bat somewhere where baseball bats aren't expected to be. (note: Don't do this, seriously. Aggravating cops isn't as fun as they make it out to be on TV)
People have a tendency to think the worst when they are given a chance to, and weapons help this thought process right along. If a store clerk sees you have a gun on you, concealed or not, their first thought is likely that you're going to rob the place and they'll get nervous. Unless this is over-ridden by something else - say a police badge - they may even act on that fear.
What Does This All Mean?
So, what is the point of all this? Mostly to point out the three big reasons why NPCs generally have good reason to be suspicious of the PCs. I'm not trying to get you, as the GM, to dry out that source of information, but it is weird that every PC is happy and friendly. Sure, helpful could be good, especially when the PCs are armed. But, take a look at what your PCs are doing. Odds are they stick out like a sore thumb, and many societies - especially older societies like you'd find in a Fantasy RPG - stand by the old Japanese phrase: deru kui ha utareru. Translated: The nail that stands out, gets hammered.