A wise man once sat in the audience and cracked a joke. All laughed like crazy.
After a moment he cracked the same joke again and a little less people laughed this time.
He cracked the same joke again and again. When there was no laughter in the crowd, he smiled and said :
“When you can’t laugh on the same joke again and again, then why do you keep crying over the same thing over and over again.”
Source : http://www.vakil.org