I know convention is to say that we don’t want to live forever, for various reasons—because, as btko said, without death life has no meaning, because we’d get bored or overwhelmed or whatever. But, truthfully, I really would want to live indefinitely. I really enjoy living, and I can’t imagine ever getting bored with life—there are so many things in existence to enjoy!—and besides, things are constantly changing. I certainly haven’t perfected the art of living yet, and forever seems like a good amount of time to work on it.
As far as aging goes, I would love to stay young for a long time—certainly longer than I am going to stay young. I like having this body and being able to abuse it in various adventurous ways that I’m sure won’t be accessible to me some day. But someday I’d also like to experience what it’s like to be old. Hard to say when that point would come.
For me, both of these things are a matter of not having enough time to experience things fully before life drags me on to the next bit. I’d just like more time to figure out how things work in this bit and enjoy them before moving on.
I don’t really believe there is any “why” to death or aging. It’s just a fact. As marina said, it’s hard to remove it from the biology. It’s like asking why the sun rises in the morning or why gravity exists. They don’t need a “why”—they have a “how” and that’s just the way things are.