I would love to live to 120 or so, soley for the chance to see us go into space and colonize other planets. In fact, I seriously want to be one of the first people to colonize Mars. Depending on when that becomes a reality (most likely 30+ years from now) they're gonna need to see the effects of space travel and exposure to alien environments on older bodies. I'd donate my body for that cause in a heartbeat.
But to answer the original intent of the question, I would only want to live as long as I wasn't a burden on my family or society. Oh wait a minute, I guess I already qualify for that...
