It's the alienation of modern living. Helping and getting help from perfect strangers is the most natural thing in the world for humans. We get a buzz off the altruism. They say the kindest thing you can do for someone is to accept their help.
In this modern society we've created there's this artificial sense that there's no social fabric, only stranger danger and it's bullshit. 97% of everyone is basically a good person being buffeted about by situational factors, so maybe it's hard to see that they're good. Just hard enough that you're never sure if you're seeing one of those 1 in 40 people who happen to be a pure monster.
The scariest threat is an ambiguous one, so it's not hard to see why we're like this - but it's much better for you and society if you can find a way to trust and lend a hand with each other. Hell, some of them will even tell you their name and remember you next time. Sooner or later, you've made a friend.
There's always the rare actually horrible person out there to be aware of, but most of us are basically fine I think.