Distrusting one another is a simple technique that helps the average person survive day-to-day stresses. I have gone through some "group" projects in which I literally do everything, and yet everyone else shares my credit, but doesn't lose the same amount of hair in the process. If I were to, in this example, trust that everyone would do their part, I would be horribly stressed that my grade had gone down for something I had no control over. Instead of being heated over an obstacle, I maneuver my way around it before it is even an issue which is a common action people take in general.
Not trusting those around you is natural and necessary in this dog-eat-dog world. Many people, especially politicians use this to their advantage in manipulation. As to with everything, there are limits to how untrustworthy one should be with others. I would say, never just believe that another person could do you wrong, but don't be surprised when it does happen because chances are, they will.