The hardest job there is
If you can talk with crowds and keep your virtue,
Or walk with Kings, nor lose the common touch;
If neither foes nor loving friends can hurt you,
If all men count with you, but none too much;
If you fill the unforgiving minute
With sixty seconds' worth of distance run,
Yours is the Earth and everything in it,
And what's more - you'll be a Man, my son!
-- From "If-" by Rudyard Kipling
Why do such men do it? That is simple: because there is no greater reward for a father than to see his son carry on his name forward through time.