> Evolutionarily the argument goes that humans are predisposed towards cooperation (i.e. good) which also seems to be backed up by archeological evidence (e.g. unremarkable individuals clearly being cared for up to old age despite evident disabilities or injuries that would have made them unable to contribute to the group's survival).
Animals developed cooperative behaviors because selective pressures rewarded the development of these behaviors for populations, but I don't think this helps us assume anything about one person's predisposition for cooperation. We can make assumptions about behavior for an individual on average based on their past actions and assumed personality characteristics. I'd like to believe that it means something to be human (ideologically, philosophically, morally, ethically etc.), but it's hard to judge what it means for other people or another person when their decision making is distorted by either past trauma or cognitive biases (I fall to this myself). Ideally, one should reciprocate the behavior one is afforded. But as you said, societal pressures sometimes do make good behavior harder to afford.
For organ donation most people donate organs because they want their death to mean something, or they do it while they're living because they make the assumption that they probably won't need two kidneys or a whole liver during the rest of their life (bad assumption given the standard American lifestyle and diet, maybe non-Americans might have better luck here). If they want to do it, whatever. For most people, a relatively healthy body and mind is their most useful asset..
I'll never encourage someone to donate organs while they're living, and even less after their death because they lose autonomy over their body. Why does post-death autonomy matter? For sentient, conscious and intelligent beings post-death autonomy is an extension of their pre-death conciousness. The rights of an individual don't and shouldn't end at their death, I think. If we don't help eachother in life and live selfishly, I don't see how and why death changes that? If the idea is that doing selfless deeds will beget more selflessness..um not in my experience, sadly. There's an entrenched individualistic tendency (almost bordering on solipsism) which often makes the receiver of the selfless deed believe that "they got something good because they're special", and not that someone else did something selfless. I think this individualistic tendency also prevents "passing it on", as you were hoping the receiver of a selfless deed might do. So I think for organ donation, if every reciever is automatically and irrevocably enrolled in organ donation themselves, then I wouldn't mind donating organs or encouraging other people to do it.