Cost of Mistrust
The decreased trust in others is both the reason to and result of decreased participation in communal activities. Without trust, we have to find costly ways to formalize our relationship(eg. creating a contract).
In a trusting society people will follow social norms for the sake of social capital. Gossip is a tool to ensure people follow the rules.
Generalized Reciprocity: If you do something good for another person, you don't need the reward immediately - eventually something good will come your way. If we don’t have to balance every exchange instantly, we can get a lot more accomplished. Trustworthiness lubricates social life.
If you want people to trust you, the best place to start is by trusting them.