Had a couple of these over the years. Shakes you up badly. Rather than dwell on it, I remind myself that it is a sign to live a full life and do what you can while you can. You don't know when it is your time, so fill your boots.
As The Flaming Lips tell us:
Do You Realize – that everyone you know someday will die
And instead of saying all of your goodbyes – let them know
You realize that life goes fast
It's hard to make the good things last