Death. 

The way I view it, you live life for two things: for yourself and your loved ones. Losing most of your mental and maybe even your physical abilities is pretty much a goodbye to all your dreams and ambitions and everything you wanted to accomplish. So the first one's already crossed out.

The other is a little more difficult. Maybe your family and even your SO would say that they loved you just as much before the accident. But that wouldn't be true. That ‘you’ they fell in love with one way or another is...well...dead. What they would remain with is but a husk. And I wouldn't want to taint their cherished memories of me by forcing them to live 20 years with this husk.

Philosophy or psychology?