Consider Eliezer's "proof" that he wants to live forever: "I want to live one more day. Tomorrow I will still want to live one more day. Therefore I want to live forever, proof by induction on the positive integers."
The breakdown, of course, is the belief that all tomorrows are the same. Some people realise that youth is temporary, and so don't look forward too much to being 70, because as infirmities crop up life gets less pleasant until they could take it or leave it. So, they like life- when life is good, and realize that life won't always be good. They also view life extension in terms of time-discounted pleasure, and so if they have to regularly starve themselves in order to live longer when they're 70, they won't because overall life pleasure will be lower.
As a young person, I am shocked and horrified by the idea of being 70.
Yet I suspect that when I am 70, I will want to live one more day.
From Mike Darwn's Chronopause, an essay titled "Would You Like Another Plate of This?", discussing people's attitudes to life:
Conclusion, graphs, and references in article. As usual, I recommend reading Chronopause.com as Darwin has many good articles; to quickly link a few: