Peter tries to keep Jesus from dying. Why was this story told?

We chat about the story of Peter trying to keep Jesus from going to Jerusalem.  Everyone knows what will happen to him if he goes there.  Even Jesus knows what will happen to him.  He will be arrested, tried, and executed.  Naturally, Peter tries to stop him from going.  Jesus calls him a stumbling block, and tells him that he is thinking about human things and not divine things.  We chat about why this story was told among early Jesus followers, and why Matthew may have included it in his Gospel account.