Might I also point out that the same irrational reasoning occurs when we pick mates? Our partners will probably share similar characteristics as many other people in the world, but the act of picking a partner immediately makes them more special than any of the thousands who may be just as pretty/intelligent/smart.
Organ donation is a tricky thing, and people don't think rationally when confronted with the death of a loved one.
I'm from Singapore, where we're automatically registered as organ donors and the majority of us are cremated after death, so organ donation shouldn't really be that much of an issue.
Sadly(?), medical science has advanced to the point where we can be kept "alive" despite being brain dead, and it is from these corpses that the organs with the best chance of a successful transplant can be obtained. It's hard to expect a family to accept organ donation when they can see that the loved one still has a heartbeat, even if the heartbeat is produced with the aid of life-support machines.
If the hospital takes a "screw you, you're stupid and we're taking your organs" attitude, the inevitable backlash has no winners and the law will end up changed. It took a lot of cajoling from our governmental mouthpieces to soothe public sentiment when that happened.