If we could live forever, than we couldn't have any children (our reproduction rate is exponential, but there's only one planet that can hold us all and it doesn't grow any larger).

Stretching this, there would be no real notion of parents or grandparents. And it would get pretty boring too.

As most of us here, you're probably too young. Do you have children? Wait until you see your first toddler walking towards you while laughing.

It's a sentiment worth dying for ;)