Ever thought that eventually the universe will end and everything humanity has ever done will be destroyed and forgotten. Also when you die say about 50 years later the majority of people will be forgotten, not even known by their great grandchildren. Finally, eventually the human race will end and everyone an everything you ever loved will be gone, only to be found by the next sentient being to inhabit the earth/planets humanity extends to (if the earth hasn't been destroyed by then!).