So let's say you want to use game theory as an "intuition pump" (colloquial meaning of that, not the original meaning) for human relationships.

We need to start by defining what "cooperate" and "defect" could actually mean in such a context. There are actually 2 obvious choices.

(I will pause here to say do not actually do this except as a fun intellectual game. People don't like calculators. Be painfully genuine and don't try to commit any of this to memory, except what sticks out.)

(I'm serious!)

The "loose" choice is:

* Defect is when the other person is behaving in a way, over a period of time, that I actively dislike.
* Cooperate is otherwise.

The "tight" choice is:

* Cooperate is when the other person is behaving in a way, over a period of time, that I actively enjoy.
* Defect is otherwise.

This might sound like splitting hairs. But you /are/ reading a thread about applying a mathematical object to the messiest domain there is, so I beg your patience. I think this initial choice has serious ramifications.

See, working with the "tight" choice probably means most people in your relationships are "defecting" against you, most of the time. Active cooperation needs to be actively fostered.

We can work back from the fact that we like most people and get a lot out of them *even though they're defecting* to conclude that, as far as we're concerned, this is a heavily positive-sum game for us.

The majority of people intuitively play games like this:

* Start out cooperative.
* If the other person cooperates, great! Keep doing whatever we were already doing.
* If the other person defects, oh no! Quick, switch strategies! M-maybe we can appease them!

But mistakes frequently happen in all 3 of these places. It's a simple fact of life. You run into someone on a bad day - he's icy and unwelcoming. You have a friend who accidentally mentioned a sore spot in your psyche - but you didn't jab them back so they never even realize it.

Notice, however, how I have just shifted the frame here. That is all true, but that is NOT thinking in the "tight" frame.

That is thinking in the "loose" frame. Defect = people actively going against our wishes.

(These threads never get much action, so I'm going to leave the rest of it as a sketch for you to fill in. Hopefully your intellectual empathy has picked up enough of how I'm cutting a path through the forest here to come to some fun conclusions yourself. Have fun! )