People grasp at things for their own imagined convenience and comfort; they grasp at wealth and treasure and honors; they cling desperately to mortal life.
They make arbitrary distinctions between existence and non-existence, good and bad, right and wrong. For people, life is a succession of graspings and attachments, and then, because of this, they must assume the illusions of pain and suffering.
Once there was a man on a long journey who came to a river. He said to himself: "This side of the river is very difficult and dangerous to walk on, and the other side seems easier and safer, but how shall I get across?" So he built a raft out of branches and reeds and safely crossed the river. Then he thought to himself: "This raft has been very useful to me in crossing the river; I will not abandon it to rot on the bank, but will carry it along with me." And thus he voluntarily assumed an unnecessary burden. Can this man be called a wise man?
This parable teaches that even a good thing, when it becomes an unnecessary burden, should be thrown away; much more so if it is a bad thing. Buddha made it the rule of his life to avoid useless and unnecessary discussions.