Since there were things I did not get to do when I was young (like piano), I would like to live long enough to be able to do those things, and also to be healthy enough to do the things I like to do now. For example, on a hike with my friend Barbara, we were talking about how our parents were in their 70s and still reasonably healthy. I was also thinking about a guy I heard about who, at the age of 92, ran a race on Mount Tamalpais, and that at least I would like to be able to hike on SF bay area trails (assuming I'm still living here) when I'm in my 90s.
I think that is part of what motivates me to exercise regularly and diet. I can't change what happened in the past, but my current actions can affect the future in a positive way. Also, it is somewhat sobering to perform in some of the retirement homes and see people who are invalids and can't do anything for themselves any more. Finally, my recent experience with high blood pressure and an injured knee made me realize I need to take better care of my body. To the extent that genetics determines longevity, I have a pretty good chance to live into my 90s because I tend to take after my mother's side of the family, many of whom lived well into their 80s and some into their 90s. (My mother has a 91 year old aunt who is still living.)
It would be interesting if the average human lifespan was well over 100 years. I wonder how it would affect human behavior. For example, in much of Tolkien's literature, some humans wanted to live as long as Elves, but Elves became weary of immortal life.