Because these are somebody else's wishes. They're somebody else's dreams.
6 seconds sound clip from the The Goonies movie soundboard.
You can hear this line at 01:01:23 in the Blu-ray version of the movie.
Quote context
[...]
- Hey, you guys, look. It must be the old Moss Garden wishing well.
- You know, I always used to believe that when you threw money in, it turned into your wish.
- You take your coins and I'll take two coins of your coins.
- Wait, wait, wait... Stop, stop. You can't do this.
- Why?
- Because these are somebody else's wishes. They're somebody else's dreams.
- Come on, One-Eyed Willy, what does this have to do with the map?
- Is this just another one of your tricks?
- Maybe we should keep him alive just in case he isn't lying.
[...]