Technically it's a question of overpopulation and finite resources. If we all lived forever, the planet or world (one could argue in this case) would eventually be unable to support us anymore. We're struggling to support ourselves as a species at the moment, imagine how bad it would be if...
I've seen it but I think you've posted this in the wrong section. I think this topic is better suited to General Discussion rather than Roleplay Discussion and Advice. You would probably get more replies posted it there.