One of the stories in Herodotus' Histories is that as Xerxes was reviewing his immense army crossing into Europe, he was seen weeping. Asked why, he said: 'Yea, for after I had reckoned up, it came into my mind to feel pity at the thought how brief was the whole life of man, seeing that of these multitudes not one will be alive when a hundred years have gone by.'
As a student (and teacher) of history, I thank you for the reminder.