Death. It's an odd thing and as far as I know, humans are the only species that are aware of their own mortality. Not surprisingly, in addition to pondering their own death, people wonder about others and when they might die too. I've known people who's lives were cut short by illness or accident, while others, like my grandfather, made it to 100.
Why it is that some people live to a ripe old age, while others seem to have barely begun before losing their lives, is baffling. Certainly genetics seems to be play a big part. People with a strong genetic makeup are most likely to be long lived, being able to stave off what invariably takes down the less fortunate.