I think, based on what I've read on game-theory so far, that humans in general are reciprocal. That is, we are wired to reciprocate (and for that matter, even emulate) the behavior that we observe and are exposed to.

Of course, none of us operate solely on the platform of Tit-For-Tat, but in general, most of us use only slightly modified variations of the theme. You will treat someone nicely if they treat you nicely, and if they hurt you, you will try to hurt them back. When someone does something nice to you, you might feel indebted, while if someone wrongs you you might want revenge. On a level playing field, Tit-For-Tat is a proven tactic.

Of course, it's not a given that the playing field is always level. Every now and then an outside factor skews it in favor of some other tactic, or even just some specific variation of the regular one, and that becomes prevalent for a while. Eventually we learn to answer that new tactic in kind, the playing field adjusts to the bias, and becomes level once more.

Since co-operation is the outcome that has the highest net profitability for all involved parties, I think that is the direction we are moving in. The more level the playing field becomes, the more collaborative we'll get.