For a minute after reading this article, it seemed unintuitive to me that anyone would find it surprising that someone else would Cooperate in their day-to-day real-life social interactions (which can be modeled as an iterated prisoner's dilemma). After all, people are supposed to more or less play Tit-for-Tat in real life, right?

I think that, in real life, there are lots of situations in which you can Cooperate or Defect to various degrees, such that sets of everyday social actions between two parties are better modeled by continuous iterated prisoner's dilemmas than by discrete iterated prisoner's dilemmas. This distinction (that continuous iterated prisoner's dilemmas model certain real life situations better than discrete iterated prisoner's dilemmas do) could be used to help explain why it feels weird when someone is really, really unusually nice, like Mark-- maybe it is normal to mostly Cooperate in social situations, but it is rare to completely Cooperate.