There's a thought experiment called the "Prisoner's Dilemma": two people are caught robbing a bank, and are taken to separate interrogation rooms; they can either rat their "friend" out or stay silent. If they both keep their mouths shut, they both take a short sentence; if one talks and the other doesn't, the rat goes free and the other guy gets a big sentence; if they both talk they both get a mid-range sentence.
The people who play this game can be neatly divided into the ones who play for the minimum jail time, and the ones who play to build trust with their friend.
Now imagine that, instead of reducing your jail time, screwing your neighbor nets you a few thousand dollars.