It is true that most people are selfish (as games designed by psychologists and anthropologists suggest so), but the fact is that humanity prospered in many ways by doing selfless things. Consider the fact that today the slavery is illegal anomaly, for example. We in the western world still have all means to take slaves as we did, by force or by other means. Also, to take away poor nation's riches - as we did in colonies.
We do not do that anymore, although it would boost our wealth. The question is - why we do not want to do that anymore?
Is it because of law? Law is a construct that can be bent either way: it is easy to make slavery legal.
Is it because of fear of retribution? It is very easy to take out guns and run over a country, mercilessly killing people, until they give up fighting (and then we can enslave them all and take their riches and pump their oilfields dry).
I think we do not do that because of our belief that we should not take advantage of other people or nations. It is contrary to our own selfish motives to get as rich as possible and to extend our power as much as possible.
Just as we are selfish, we can and we do give selflessly. It somehow drives progress, and we're now living in an era of very small number of wars compared to our past.
The catch is, we had to teach ourselves that selflessness and cooperation make best future possible.
I think it is the same with organ donation: it should be promoted, not regulated. We want people to get the information, to actually understand why it is so valuable for them and for society, we want them to willingly donate organs because they know and they understand the impact of their decision on other people and how it would make some great things even if they have no means to benefit from their decision.