Life is precious because it is unique.
Each one of us wants to hang on to it till the very last. An 86 year old is as keen to live as a 26 year year old.
Our brains, and our whole psyche is designed to relish each approaching moment. The 86 year old knows most of his life is behind him/her, and yet he/she is keen to experience more of what life has to offer. We all look to the future because most of us, if we've been lucky, have had good experiences in the past, and would like more of the same.
Great philosophers like Plato have told us that we feel the world with our senses: of sight, smell, hearing, taste, and touch. Scientific research has only confirmed this and have shown us that all this information collected by our senses is conveyed to the brain, where a perception of reality is built. It is still being debated whether knowledge is just the sum of the sensations, or knowledge happens inside our brains because of reasoning etc.
So, it is obvious that there is a strong link between our physical health (of the nerves and the brain etc.) and our sense of well being. As the Greeks of antiquity told us, a sound mind and a sound body go together!
Here is some interesting advice from a 97 year old Japanese Doctor, who is still very active in pursuing his career. His experiences bear out the saying do what you love, and you'll love what you do!
However, I dont think it is the number of years that you live that is important, but the quality of the life that you have lived.
Do you agree? And what do you think is a "good life"?
http://www.japantimes.co.jp/text/fl20090129jk.html