An extreme illustration:
Suppose you have an identical twin. For whatever reason, you've never gotten along with him. You were always very mean to him while you were growing up; you'd break his toys, blame him for bad things that you did, and tell lies about him to his friends. You even physically injured him when your parents weren't looking (pushed him down the stairs, disconnected the hand brakes on his bicycle, etc) and poisoned the tropical fish in his aquarium.
When you both turned 18 you went off to different universities and never communicated with one another. Suddenly you became very ill and after a series of medical tests it was determined that you were in the final stages of renal failure, and unless you had a kidney transplant very soon would most likely die. Of course your twin is a perfect match, but when you ask him for one of his kidneys, he laughs at you and says "After all those years of torturing me, you want a favor? And a very serious favor - one of my bodily organs?! Surely you jest!" And he hangs up the phone.
You have reaped what you have sown.
- For Teachers