What happens when someone else's wish is bestowed upon you?

Charlotte (Charlie) always wanted out of the trailer park. It was a promise she made to herself at every one of her seventeen birthdays. Twin brother Johnny, on the other hand, never considered it. A life outside of the trailer park wasn't a possibility, so he never hoped or planned for more. So, when it seems like he picks the wrong walnut shell and receives the prize that should be his sister's, it doesn't make him feel as good as it should. This is why "It's a Wonderful Life" by Sparklehorse represents Johnny and Charlie's predicament so well. The lyrics for the most part are happy, as well as the tune. However, there's a contrasting sad undertone somehow, and it represents Johnny's surprise good fortune. There are two lines that portray his dilemma perfectly.

I'm the dog that ate
Your birthday cake