I do not want to live long enough to see everyone I love die off. I'm not sure what age that is...but I really do not want to bury everyone currently in my life.
I also imagine that this world will eventually wear me down to a nub. I'm already beginning not to care about the constant shit swirl that 90% of the world is. Wars, death, taxes, politics, resources, work, weather, etc...eventually I won't want to endure another winter, or listen to yet another politician pander for votes, or hear about the latest crisis/emergency/scare, or hear doom and gloom profits tell us all how things are going to end "any day now".
In other words, eventually I'll be ready to die. I would never want to live forever.
Imagine a 115 year old person that would be alive now. Imagine all they would have went through between 1901 and now. All the wonders, all the horrors, everyone they ever loved in the ground. No thanks.