Getting to "know" a person, and sharing a social network with them, raises the cost to them of harming me. They would lose my good will, which could easily be more valuable in the long run than the contents of the bag. I could also harm their reputation by telling others about their crime, possibly costing them other friendships. There is also a filtering process in which evidence that people are likely to treat me badly leads to me not bothering to get to know them better.