If your life and self-worth is defined by just by one thing (children, work, school, money, athletic success), you are in a very fragile position.
Any change in that one thing can bring you all the way down. It is easy to see with athletes once their career is over (the scene in Friday Night Lights with Boobie crying in the car talking about "What am I going to do now?")
But it extends elsewhere. You see people go into deep depression after losing a job (even when there are no money issues) or when their children grow up and don't need them as much.