ps: Yes, basically because cooporation is advantageous and will arise spontaneously. You should worry i am making a tautology here (why is cooporation advantageous? because otherwise evolution would have breed it out. Why do we coorporate? because we evolved that way...), but this is seen again and again both in artificial life simulations (where the robots "learn" to coorporate) and in game theory.
I think the results in game theory is particulary strong. Particulary the tit-for-tat strategy arise as a nash equilibrium (nash. eq. mean that it is unfavorable for any agent to select another strategy) in a very general game:
Be generally good to others when you meet them.
Do to others what they did to you. (eg. if they coorporated, continue coorporation. If they cheated and left you out to dry, dont coorporate)
forgive when they change strategy.
Notice this is how most of us will try to act. Ofcourse this is a very crude model, in particular games with many agents where a "reputation" is involved are very prone to favor coorporative behaviour as nash equilibrium, it is interesting how "reputation" play a huge role in human society.
I am not sure how this applies to why suffering bothers us...
At what point in human evolution did we evolove into these "beliefs"?