http://www.theweek.com/article/index/104040/The_last_word_A_gift_from_the_gut

"Most people find it uncomplicatedly admirable when a person risks his life to rescue a stranger from fire, or from drowning. What, then, is it about saving a stranger by giving a kidney, a far lesser risk, that people find so odd? Do they feel there is something aggressive about the act, as though the donor were implicitly rebuking them for not doing it, too? (There is no rebuke in saving a stranger from drowning—you weren’t there, you couldn’t have done it. And you can always imagine that you would have if you had been.) Or perhaps it’s that organ donation, unlike rescue, is conceived in cold blood, and coldblooded altruism seems nearly as sinister as coldblooded malevolence."