I think it's the way we're raised and the environment we live in. We've been taught to be afraid by parents, the police, schools, the media. Geez, especially the media. In order to increase ratings they exaggerate stories and make you think that everything that happens is an epidemic and is going to kill you. For instance, after the Columbine shootings, there were dozens of stories about how school shootings were epidemic in America, even though in fact they are rare. But it increase the level of fear and mistrust.
Stranger Danger is one of my favorites. Here the media probably was trying to do some good, letting the public know that kids are missing. It became so extensive, so in your face, that parents saw kidnappers on every corner. But what are the facts: In 1999 about 69,000 children were abducted. Of those, 82% were abducted by known family members (usually due to custody disputes), 11% by friends and acquaintances, and 2-3 % by strangers. So the truth is, it's not the strangers we should have been watching out for, it's the parents!
It's things like this, the misinformation, the overload of information, which I think that has contributed most to our mistrust of each other.