Why only one change per wish?

Jon didn't know what to do with the stone. There was so many ideas along with many thoughts. "What if I mess up and can't fix it?" What if the stone tricks me?" "What if I don't get what I want?" Then a new idea popped into his head. "I wish life was like my fantasies" Suddenly Jon felt dizzy, the room blacked out. He couldn't feel, hear, or see anything, then things started to come into focus.....