You shouldn't tell your friend's secrets to others.
Dan is with his friend Mark who tells Dan that he has a crush on a girl in school.
Dan wants to get the girl together with Mark.
Dan makes sure to talk about Mark's good qualities with the girl at every possible opportunity.
Dan walks up to the girl and tells her that Mark is interested in her.
The girl becomes attracted to Mark and starts dating him.
The girl tells her friends, the whole school finds out, and Mark is very embarrassed.