As a medical professional, I can't stress how important it is to donate your organs. There are people every single day in need of various organs and many are unable to obtain one not just because of availability, but also tissue compatibility and rejection. Thus, the greater number of organs available, the greater the chance of a match. If you are considering to do so, please do.
There is one thing I've noticed in this thread however, and that's a startling amount of judgement going on. People are free to do whatever they like with their bodies, whether they are alive or dead. It's their body, even in the hospital we have laws and rules when we deal with recently deceased patients. If a patient wants to keep their organs for whatever reason, they are absolutely free to do so. But labelling people as "selfish" or "treating humanity like shit" because they choose not to is not right. In the medical community, we respect and observe that each patient has the right to have their body treated however they would like to. Just because they have passed on doesn't mean they owe society any part of their body.
Organ donation should be an act of kindness, generosity, and compassion as opposed to being shamed into participating.
The best way we can encourage each other to be organ donors is through education and meaningful conversation. Demonizong those who see differently is not the answer.