Matt Snyder has a really good article up on CBS Sports about Jake Arrieta's emergence as an ace and total domination over the course of the season. I'll just quote the ending, which doesn't give anything away that you didn't already know so go read the article.
Quite simply, Jake Arrieta — who a contender had no use for two years ago — is now having the best season any Cubs pitcher has ever had and the best second half anyone has ever had. What an amazing game baseball can be.